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SUMMARY TECHNICAL REPORT 
OF THE 

NATIONAL DEFENSE RESEARCH COMMITTEE 


This document contains information affecting the national defense of the 
United States within the meaning of the Espionage Act, 50 U. S. C., 31 and 
32, as amended. Its transmission or the revelation of its contents in any 
manner to an unauthorized person is prohibited by law. 

This volume is classified RESTRICTED in accordance with security 
regulations of the War and Navy Departments because certain chapters 
contain material which was RESTRICTED at the date of printing. Other 
chapters may have had a lower classification or none. The reader is advised 
to consult the War and Navy agencies listed on the reverse of this page 
for the current classification of any material. 





Manuscript and illustrations for this volume were prepared for 
publication by the Summary Reports Group of the Columbia 
University Division of War Research under contract OEMsr-1131 
with the Office of Scientific Research and Development. This vol¬ 
ume was printed and bound by the Columbia University Press. 

Distribution of the Summary Technical Report of NDRC has 
been made by the War and Navy Departments. Inquiries concern¬ 
ing the availability and distribution of the Summary Technical 
Report volumes and microfilmed and other reference material 
should be addressed to the War Department Library, Room 
1A-522, The Pentagon, Washington 25, D. C., or to the Office 
of Naval Research, Navy Department, Attention: Reports and 
Documents Section, Washington 25, D. C. 


Copy No. 

318 


This volume, like the seventy others of the Summary Technical 
Report of NDRC, has been written, edited, and printed under 
great pressure. Inevitably there are errors which have slipped 
past Division readers and proofreaders. There may be errors of 
fact not known at time of printing. The author has not been able 
to follow through his writing to the final page proof. 

Please report errors to: 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D. C. 

A master errata sheet will be compiled from these reports and 
sent to recipients of the volume. Your help will make this book 
more useful to other readers and will be of great value in prepar¬ 
ing any revisions. 


RLSTiur/m* 




SUMMARY TECHNICAL REPORT OF DIVISION 16, NDRC 


VOLUME 3 


NON-IMAGE FORMING 
INFRARED 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 

NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 

DIVISION 16 

GEORGE R. HARRISON, CHIEF 


WASHINGTON, D. C., 1946 





NATIONAL DEFENSE RESEARCH COMMITTEE 


James B. Conant, Chairman 
Richard C. Tolman, Vice Chairman 
Roger Adams Army Representative 1 

Frank B. Jewett Navy Representative 2 

Karl T. Compton Commissioner of Patents 3 

Irvin Stewart, Executive Secretary 


1 Army representatives in order of service: 


Maj. Gen. G. V. Strong 
Maj. Gen. R. C. Moore 
Maj. Gen. C. C. Williams 
Brig. Gen. W. A. Wood, Jr. 

Col. E. A. 


Col. L. A. Denson 
Col. P. R. Faymonville 
Brig. Gen. E. A. Regnier 
Col. M. M. Irvine 
Routheau 


2 Navy representatives in order of service: 

Rear Adm. H. G. Bowen Rear Adm. J. A. Furer 

Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 

Commodore H. A. Schade 
3 Commissioners of Patents in order of service: 
Conway P. Coe Casper W. Ooms 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suitable 
projects and research programs on the instrumentalities of 
warfare, together with contract facilities for carrying out 
these projects and programs, and (2) to administer the tech¬ 
nical and scientific work of the contracts. More specifically, 
NDRC functioned by initiating research projects on re¬ 
quests from the Army or the Navy, or on requests from an 
allied government transmitted through the Liaison Office 
of OSRD, or on its own considered initiative as a result of 
the experience of its members. Proposals prepared by the 
Division, Panel, or Committee for research contracts for 
performance of the work involved in such projects were 
first reviewed by NDRC, and if approved, recommended to 
the Director of OSRD. Upon approval of a proposal by the 
Director, a contract permitting maximum flexibility of 
scientific effort was arranged. The business aspects of the 
contract, including such matters as materials, clearances, 
vouchers, patents, priorities, legal matters, and administra¬ 
tion of patent matters were handled by the Executive Sec¬ 
retary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 
These were: 

Division A—Armor and Ordnance 

Division B—Bombs, Fuels, Gases, & Chemical Problems 
Division C—Communication and Transportation 
Division D—Detection, Controls, and Instruments 
Division E—Patents and Inventions 


In a reorganization in the fall of 1942, twenty-three ad¬ 
ministrative divisions, panels, or committees were created, 
each with a chief selected on the basis of his outstanding 
work in the particular field. The NDRC members then be¬ 
came a reviewing and advisory group to the Director of 
OSRD. The final organization was as follows: 

Division 1—Ballistic Research 

Division 2—Effects of Impact and Explosion 

Division 3—Rocket Ordnance 

Division 4—Ordnance Accessories 

Division 5—New Missiles 

Division 6—Sub-Surface Warfare 

Division 7—Fire Control 

Division 8—Explosives 

Division 9—Chemistry 

Division 10—Absorbents and Aerosols 

Division 11—Chemical Engineering 

Division 12—Transportation 

Division 13—Electrical Communication 

Division 14—Radar 

Division 15—Radio Coordination 

Division 16—Optics and Camouflage 

Division 17—Physics 

Division 18—War Metallurgy 

Division 19—Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 



IV 



NDRC FOREWORD 


A s events of the years preceding 1940 revealed 
- more and more clearly the seriousness of the 
world situation, many scientists in this country 
came to realize the need of organizing scientific re¬ 
search for service in a national emergency. Recom¬ 
mendations which they made to the White House 
were given careful and sympathetic attention, and 
as a result the National Defense Research Commit¬ 
tee [NDRC] was formed by Executive Order of the 
President in the summer of 1940. The members of 
NDRC, appointed by the President, were instructed 
to supplement the work of the Army and the Navy 
in the development of the instrumentalities of war. 
A year later, upon the establishment of the Office 
of Scientific Research and Development [OSRD], 
NDRC became one of its units. 

The Summary Technical Report of NDRC is a 
conscientious effort on the part of NDRC to sum¬ 
marize and evaluate its work and to present it in a 
useful and permanent form. It comprises some 
seventy volumes broken into groups corresponding 
to the NDRC Divisions, Panels, and Committees. 

The Summary Technical Report of each Division, 
Panel, or Committee is an integral survey of the 
work of that group. The first volume of each group’s 
report contains a summary of the report, stating the 
problems presented and the philosophy of attacking 
them, and summarizing the results of the research, 
development, and training activities undertaken. 
Some volumes may be “state of the art” treatises 
covering subjects to which various research groups 
have contributed information. Others may contain 
descriptions of devices developed in the laboratories. 
A master index of all these divisional, panel, and 
committee reports which together constitute the 
Summary Technical Report of NDRC is contained 
in a separate volume, which also includes the index 
of a microfilm record of pertinent technical labora¬ 
tory reports and reference material. 

Some of the NDRC-sponsored researches which 
had been declassified by the end of 1945 were of 
sufficient popular interest that it was found desir¬ 
able to report them in the form of monographs, such 
as the series on radar by Division 14 and the mono¬ 


graph on sampling inspection by the Applied Mathe¬ 
matics Panel. Since the material treated in them 
is not duplicated in the Summary Technical Report 
of NDRC, the monographs are an important part 
of the story of these aspects of NDRC research. 

In contrast to the information on radar, which is 
of widespread interest and much of which is released 
to the public, the research on subsurface warfare is 
largely classified and is of general interest to a 
more restricted group. As a consequence, the report 
of Division 6 is found almost entirely in its Sum¬ 
mary Technical Report which runs to over twenty 
volumes. The extent of the work of a division can¬ 
not therefore be judged solely by the number of 
volumes devoted to it in the Summary Technical 
Report of NDRC; account must be taken of the 
monographs and available reports published else¬ 
where. 

Division 16 carried out a broad program in the 
fields of light and optics. Among the studies under¬ 
taken were a number involving the principles and 
techniques of camouflage, and perhaps the outstand¬ 
ing success achieved in this field was the develop¬ 
ment of the “black widow” finish for night-flying 
aircraft. Significant improvements were made in 
aerial mapping and photography. Devices depend¬ 
ing on the use of infrared light were developed for 
the detection of enemy craft, the recognition of 
friendly ones, and for intercommunication by voice 
and code. The sniperscope, using image-forming 
infrared rays, was a spectacular weapon which en¬ 
abled our troops to fire accurately on an enemy 100 
yards away in utter darkness. 

The Division 16 Summary Technical Report, pre¬ 
pared under the direction of the Division Chief, 
George R. Harrison, describes the technical achieve¬ 
ments of the Division personnel and its contractors, 
and is a record of their skill, integrity and loyal 
cooperation. To all of them, we extend our grateful 
praise. 

Vannevar Bush, Director 
Office of Scientific Research and Development 
J. B. Conant, Chairman 
National Defense Research Committee 




v 




FOREWORD 


A t the time of its formation late in 1942, Divi- 
- sion 16, the Optics Division of NDRC, was 
assigned both the general task of stimulating and 
supervising OSRD research in optics and the imme¬ 
diate problem of overseeing a large number of con¬ 
tracts which had previously been initiated by the 
Instruments Section. Inasmuch as the new Division 
consisted to a large extent of personnel associated 
with the Instruments Section during 1940 and 1941, 
the reorganization involved few important changes. 

The present Summary Technical Report describes 
the accomplishments of both Division 16 and Sec¬ 
tion D-3, and covers the principal developments in 
optics made in America during World War II. This 
report should be considered as intermediate in char¬ 
acter between the detailed contractors’ reports of 
Division 16, to which reference is frequently made 
herein which are complete scientific reports of the 
investigations carried on, and the historical volume 
entitled Optics and Applied Physics in World War 
II, which presents in less technical form the accom¬ 
plishments of the Division and its contractors, and 
assigns credit to those who took part. 

The contents of the present volume demonstrate 
impressively the great contribution made by the 
optical industry of America and the university opti¬ 
cal laboratories to the war effort. While less glamor¬ 
ous than some of the newer fields brought into 
existence during the war, optics nevertheless made 
significant contributions which were by no means 
confined to mere extension or application of optical 
methods or apparatus previously in use. The stress 
of the emergency produced many new optical de¬ 


velopments, and the genesis of a large proportion 
of these will be found recorded in the following 
pages. 

The science of optics and the optical industry 
have both benefited greatly by the intensive re¬ 
search which took place during the war. Many of 
the new devices developed under emergency condi¬ 
tions have contributed and will contribute more to 
our fundamental understanding of optics, and many 
of them will have peacetime applications. New lines 
along which optical research should be directed 
have been made apparent. In particular, the infra¬ 
red field has benefited greatly, and the art of in¬ 
frared phosphor development and utilization has 
been elevated to an entirely new level. 

Consideration of the developments in optics, as 
in other fields, emphasizes that, once adequate im¬ 
mediate defense has been insured, more important 
than having weapons for a possible future war is 
having available a large body of trained personnel 
who can step into any breach that occurs and be 
available to produce the new devices that may be 
needed. 

The Optics Division of NDRC is especially in¬ 
debted to the chiefs and members of its Sections, 
whose names are listed at the end of this volume. 
They have provided the essential leadership, com¬ 
bined with scientific knowledge, without which the 
work of the Division could not have been planned 
or completed. 

George R. Harrison 
Chief, Division 16 


vii 






PREFACE 


T his volume of the Summary Technical Report 
of Division 16, NDRC, records the essential 
features of the scientific and technical develop¬ 
ments carried out under the auspices of Section 16.4 
for the military use of infrared radiation. The 
developments described herein are limited to the 
field of non-image forming infrared equipments, 
since the image-forming ones are described in Vol¬ 
ume 4 of Division 16 STR. 

It is the purpose of this volume to furnish for 
qualified technical personnel of the Armed Services, 
and for their future civilian scientific and technical 
collaborators, a condensed objective record of the 
problems and achievements of the NDRC in this 
field, with primary emphasis given to the develop¬ 
ments completed under the auspices of Section 16.4 
from its formation in January 1943 to the termina¬ 
tion of experimental work during the autumn of 
1945. It is intended as an outline of the principal 
achievements and a guide to the more comprehen¬ 
sive accounts contained in the contractors’ reports 
listed in the bibliography. 

Some of the earlier work carried out in this field 
under sections D-3 and D-4, NDRC, which formed 
a partial basis for the later developments described 
herein, and also certain related, subsidiary devel¬ 
opments made under Section 16.4 are mentioned 
only in passing as background material. Detailed 
accounts of these may be found if they should be 
wanted in the bibliographical references. Certain 
infrared components closely related to those devel¬ 
oped by Section 16.4 and employed in military 
equipments by other divisions, for example, the heat¬ 
homing missiles developed by Division 5, have been 
left for treatment in the Summary Technical Report 
volumes of those divisions. In other instances brief 
descriptions of similar or related components or 
equipments developed in the United States under 
other than NDRC auspices, by the British, or by 
the enemy, are given for comparative purposes. Such 
accounts have been kept to a minimum, however, 
in accordance with the basic premise that the report 
of each section of NDRC should reflect essentially 
only the developments for which that section was 
primarily responsible. 

For military purposes, infrared radiation may be 
divided into near infrared [NIR] extending from 
the visible region to a wavelength of about 5.5 
microns, and far infrared [FIR], extending from 
5 to 15 microns in wavelength. Because of the 
radiation-transmission properties of long paths 
through the atmosphere as well as the characteris¬ 


tics of the only near infrared radiation detectors 
which were available until the last phases of World 
War II, the wavelength regions actually utilized 
extended only from the visible region to about 1.5 
microns in the near infrared region, and essentially 
from 8.5 to 13 microns in the far infrared region. 

Generally speaking, the principal difference be¬ 
tween the systems used in the near and far infrared 
regions lies in the fact that most near infrared 
devices require the use of a special source of near 
infrared radiation directed toward the receiver, 
whereas the target itself is a self-luminous source 
for the far infrared devices. The near infrared sys¬ 
tems employ a photoelectric or photoconductive 
radiation detector having selective spectral response 
characteristics, while the far infrared devices uti¬ 
lize a detector which absorbs, non-selectively at 
all wavelengths, the emitted radiant energy as 
heat. 

On account of these differences, this volume may 
be regarded as having four fairly well-defined parts. 
The first three chapters describe the characteristics 
of the three essential optical components—sources, 
filters, and detectors—of non-image-forming sys¬ 
tems working in the near infrared region. Chapters 
4 to 7, inclusive, describe the general construction, 
properties, and military applications of such sys¬ 
tems for communication by voice and by code, 
recognition and identification, determination of the 
range and direction of targets, and indication of the 
position of a glider with respect to that of its tow 
plane. The generalized photometric nomenclature 
outlined in OSRD Report 1585, which is attached 
as an appendix to this volume, is used throughout 
these chapters. This terminology was developed 
for this spectral region by Section 16.4 and was 
adopted by the Combined Communications Board 
of the Combined Chiefs of Staffs representing both 
the United States and Great Britain. 

Chapter 8 outlines the development and charac¬ 
teristics of several different kinds of thermal detec¬ 
tors for far infrared radiation. In Chapter 9 are 
described the construction, characteristics, and mili¬ 
tary applications of far infrared receiving equip¬ 
ments employing such detectors for the detection 
of personnel, vehicles, marine craft, military mate¬ 
riel, factories, power plants, etc., for the determina¬ 
tion of range and direction of marine craft, for the 
thermal mapping of terrain from aircraft, and for 
the guidance to their targets of certain heat-sensi¬ 
tive, target-seeking missiles. 

In the preparation of this volume, emphasis has 


ix 


X 


PREFACE 


been given to the properties of the fundamental 
components, to the types of possible military appli¬ 
cations and to the characteristics of the equipments 
developed for these applications. It is felt that this 
type of treatment will give the qualified technical 
reader a clear view of the present technical status 
of the development and use of such components and 
equipments and will provide a definite point from 
which further developments, some of which are pro¬ 
posed herein, may stem without duplication of past 
effort. A detailed account of the steps taken in 
these developments and of the methods of construc¬ 
tion of the various components and systems 
sufficient to permit their reproduction has been 
avoided. 

It is appropriate at this place to acknowledge the 
contribution of the different contracting organiza¬ 
tions and of the individual research personnel and 
to extend to them the appreciation of Section 16.4, 
NDRC, for their loyal cooperation in carrying out 
the scientific and technical developments of the 
integrated program of Section 16.4 described herein. 
Following is a list of the research personnel who 
contributed significantly to the outstanding achieve¬ 
ments of each contract. It is hoped that no names 
have been inadvertently omitted. 

University of California OEMsr-1073 

Harvey E. Whi^e, Technical Representative; 

Alfred Einarsson, P. M. Harris, Charles G. Miller 
Farnsworth Television and Radio Corporation 
OEMsr-1094 

Madison Cawein, Technical Representative; 

C. C. Larson, Alfred Roman, H. Salinger 
General Electric Company OEMsr-1322 

I. F. Kinnard, Technical Representative; 

J. J. Fitz Patrick, C. W. Hewlett, H. T. Wrobel 
Harvard University OEMsr-60 

E. L. Chaffee, Technical Representative; 

F. G. Blake, Jr., C. P. Butler, A. C. Grant, G. L. 

Harvey, J. R. Hooper, Jr., N. C. Jamison, R. A. 

Mack, J. D. Strong, J. L. Winget 

Johns Hopkins University OEMsr-610 

A. H. Pfund, Technical Representative; 

W. G. Fastie 

Massachusetts Institute of Technology NDCrc-180 

A. C. Bemis, Technical Representative; 

Willard E. Buck, Ernest B. Dane, A. W. Friend, 

Roy W. Prince 

Massachusetts Institute of Technology OEMsr-561 

A. R. von Hippel, Technical Representative; 

E. S. Rittner, J. H. Shulman 
Massachusetts Institute of Technology OEMsr-576 

Hans Mueller, Technical Representative; 

Elias Burstein, Norman J. Oliver 


Massachusetts Institute of Technology OEMsr-1036 
A. R. von Hippel, Technical Representative; 

F. G. Chesley, H. S. Denmark, E. S. Rittner, 
P. B. Ulin 

Massachusetts Institute of Technology OEMsr-1147 

A. C. Bemis, Technical Representative; 

Louis Harris 

University of Michigan NDCrc-185 
E. F. Barker, Technical Representative; 

J. G. Black, R. W. Engstrom, A. Fairbanks, 
P. H. Ge : ger, W. L. Hole, L. N. Holland, C. V. 
Kent, T. R. Kohler, 0. G. Koppius, W. W. 
McCormick, G. A. Van Lear, Jr. 

University of Michigan OEMsr-1132 
E. F. Barker, Technical Representative; 

L. W. Gildart 

Northwestern University OEMsr-235 

B. J. Spence, Technical Representative; 

R. J. Cashman 

Northwestern University OEMsr-990 
B. J. Spence, Technical Representative; 

W. S. Huxford, E. W. Lothrop, R. L. Osborn, 
J. R. Platt, H. S. Snyder, W. R. Wilson 
Northwestern University OEMsr-1391 
B. J. Spence, Technical Representative; 

R. W. Hummer 

Ohio State University OEMsr-987 
Wallace R. Brode, Technical Representative; 

E. S. Hodge, R. T. Morris, E. E. Pickett, J. H. 
Shenk 

Ohio State University OEMsr-1168 
Harald H. Nielsen, Technical Representative; 

E. E. Bell, A. H. Nielsen 
Polaroid Corporation OEMsr-1085 

E. H. Land, Technical Representative; 

W. F. Amon, Jr., E. R. Blout, Ridgley D. Shep¬ 
herd, Jr., Alexander Thomas 
Radio Corporation of America OEMsr-1486 
Dayton Ulrey, Technical Representative; 

R. W. Engstrom, A. C. Glover 
V-M Corporation OEMsr-1460 

F. Smardo, Technical Representative 
Western Electric Company OEMsr-636 

J. A. Becker, Technical Representative; 

W. H. Brattain, H. Christensen, L. M. Ilgenfritz, 
J. B. Johnson, J. B. Kelly, R. W. Ketchledge, 
J. J. Kleimack, W. A. MacNair, H. R. Moore, 
N. G. Wade 

Western Electric Company OEMsr-1098 
J. A. Becker, Technical Representative; 

W. H. Brattain, H. Christensen, L. M. Ilgenfritz, 
H. R. Moore 

Western Electric Company OEMsr-1231 

G. K. Teal, Technical Representative; 

E. F. Kingsbury, A. W. Treptow 






PREFACE 


xi 


Western Electric Company OEMsr-1267 

R. C. Mathes, Technical Representative; 

E. Bruce, W. Herriott, E. F. Kingsbury 
Wester^ Union Telegraph Company OEMsr-984 

H. P. Corwith, E. C. Homer, Technical Repre¬ 
sentatives ; 

W. D. Buckingham, C. R. Deibert, H. L. Smith 

It is also a distinct pleasure for the editor to 
acknowledge the indebtedness of Section 16.4 to 
those individuals whose painstaking devotion to a 
laborious task made possible the creation of this 
record. These include Dr. Winston L. Hole, author 
of Chapters 1 and 6, and an invaluable critic and 
assistant in the overall layout and editing of the 
volume; Dr. Richard C. Lord, author of Chapter 2, 
who participated in the initial planning of the vol¬ 
ume and assisted in the editing of some of the 
chapters; Dr. John R. Platt, author of Chapters 3, 


4, and 5; Mr. Thomas R. Kohler, author of Chapter 
7; Dr. Harald H. Nielsen and Dr. Alvin H. Nielsen, 
co-authors of Chapters 8 and 9; and Mr. Charles A. 
Federer, Jr., who cooperated in the copy-editing of 
the manuscripts and performed general liaison duty 
between Division 16 and the Summary Reports 
Group at Columbia University. 

Acknowledgment is also made to the different 
contractors for furnishing the master copies of most 
of the illustrations, and to the Army and Navy for 
permission to use certain illustrations. 

Finally, the invaluable cooperation of Dr. O. S. 
Duffendack, Chief of Section 16.4, NDRC, in plan¬ 
ning the volume, his helpful suggestions concerning 
its scope and contents throughout its preparation, 
and his careful reading of the manuscript are greatly 
appreciated. 

James S. Owens 
Editor 














CONTENTS 


CHAPTER PAGE 

Summary of Developments in Infrared Techniques, Com¬ 
ponents, and Equipments under the Auspices of Section 16.4 
NDRC by 0. S. Duffendack . 1 

1 Near Infrared Sources by Winston L. Hole . 9 

2 Near Infrared Transmitting Filters by Richard C. Lord ... 45 

3 Non-Image-Forming Near Infrared Detecting Devices by 

John R. Platt . 55 

4 Near Infrared Voice-Code Communication Systems by John 

R. Platt . 93 

5 Near Infrared Recognition and Code Communication Sys¬ 
tems by John R. Platt . 170 

6 Near Infrared Systems for Detecting Range and Direction 

[IRRAD] by Winston L. Hole . 200 

7 Glider Position Indicator and Cloud Attenuation Meter by 

Thomas R. Kohler . 215 

8 Far Infrared Detecting Elements by Harald H. Nielsen and 

Alvin H. Nielsen . 225 

9 Far Infrared Receiving Systems for Military Applications 

by Alvin H. Nielsen and Harald H. Nielsen . 279 

Appendix . 363 

Glossary . 369 

Bibliography . 373 

OSRD Appointees. 381 

Contract Numbers . 384 

Service Project Numbers. 385 

Index . 387 


xiii 


























SUMMARY OF DEVELOPMENTS IN INFRARED TECHNIQUES, COMPONENTS, 
AND EQUIPMENTS UNDER THE AUSPICES OF SECTION 16.4, NDRC 

By O. S. Duffendack a 


SCOPE AND NATURE OF DEVELOPMENTS 

S ection 16.4 of the National Defense Research 
Committee [NDRC] was charged with the 
development of infrared equipments which do not 
require the formation of a visual image of the target 
but reveal the presence of the target by aural sig¬ 
nals or by indicated signals on flashing lamps, 
cathode-ray oscilloscopes, meters, or recorder charts. 

The work was divided into two major parts: 
one part using near infrared radiation, from about 
0.8 [i to 1.5 p, and the other part using far infra¬ 
red radiation, from about 5 p to 15 p. The techniques 
and apparatus used in the two parts are very differ¬ 
ent. In the near infrared, it is necessary to use an 
appropriate source of radiation, and rays from this 
source are detected by the receiver, either directly 
along a geometric beam or indirectly after regular 
or diffuse reflection. With far infrared, the target 
itself is the source, being self-luminous because of 
the thermal radiation emitted characteristic of its 
temperature. In the intermediate infrared from 
about 1.5 to 5.5 p, both types of techniques are pos¬ 
sible and both were under development at the con¬ 
clusion of World War II. 

Near infrared devices, “Nancy” equipment, were 
developed for purposes of (a) detecting and locat¬ 
ing other installations of Nancy equipment, friendly 
and enemy planes, etc., (b) ranging, (c) recogni¬ 
tion or identification of friendly ships and planes, 
and (d) communication by voice and by code. The 
most extensive developments were for ship recog¬ 
nition and for voice and code communication be¬ 
tween ships. The range of these equipments in 
average clear weather is 6.5 to 10 miles; the distance 
for effective operation decreases rapidly with in¬ 
creasing cloudiness or fog. 

Infrared systems were developed primarily for use 
under military conditions in which it is desirable 
to maintain radio and radar silence. The trans¬ 
mitters used with near infrared systems, particularly 
the communication systems, project sharply defined 

a Chief, Section 16.4 NDRC; Director, Philips Labora¬ 
tories, Inc., Irvington, New York. 


beams of radiation and cannot be detected except 
by a receiver properly oriented in the beam. When 
necessary the beam can be restricted to a very small 
angle and is, moreover, limited to line-of-sight re¬ 
ception. Thus a very high measure of system secu¬ 
rity is achieved, and message security may be still 
further enhanced by special conditions of modula¬ 
tion, some of which are briefly indicated in sub¬ 
sequent paragraphs. 

Since the far infrared systems utilize only the 
natural thermal radiation from the target and do 
not require an auxiliary source, their operation is 
not self-revealing in any way, at any distance. Far 
infrared receivers were developed: (a) for the de¬ 
tection and location of personnel, vehicles, tanks, 
planes, and ships; (b) for ranging; and (c) for the 
guiding of missiles. The range of the far infrared 
receivers is from a few hundred yards for personnel 
to about 12 sea miles for a destroyer in average 
clear weather. In some of the experiments the range 
was limited by the horizon because the apparatus 
was mounted at a relatively low elevation on ship¬ 
board. Again the range is drastically reduced by 
clouds and fog. 

Intermediate infrared devices were in the earlier 
stages of development but showed great promise for 
use both in the detection of military objects and in 
communications. 

NEAR INFRARED DEVELOPMENTS 

Photocells 

In order that near infrared equipments of military 
value could be developed, it was necessary that more 
sensitive photocells for this region should be ob¬ 
tained. The only photocells of this type available 
at the beginning of the war were cesium photocells, 
and these were being manufactured only on a small 
scale. A few electron multipliers with cesium photo¬ 
cathodes were being made for laboratory use. Sec¬ 
tion 16.4 instituted two lines of research and de¬ 
velopment, one to improve the characteristics and 
production methods for improved electron multi¬ 
pliers and another to develop new types of photo- 




1 




2 


SUMMARY 


cells sensitive to infrared radiation. Both lines were 
fruitful, and the equipments developed profited by 
the availability of these improved detectors. 

Several research and development groups in dif¬ 
ferent laboratories concentrated on the problems 
involved in improving and producing cells of the 
Thalofide type, first produced by Case during World 
War I. The result of these efforts was the develop¬ 
ment of a very stable thallous sulfide photoconduc- 
tive cell which was incorporated in several military 
equipments, such as those indicated for recognition 
and communication both by voice and by code. The 
methods for the quantity productions of these cells 
in two specific types were worked out by two tube 
manufacturers. 

Sources 

Besides improved photocells for infrared receivers, 
it was necessary to develop improved sources of near 
infrared radiation. These developments proceeded 
along four lines. 

1. Tungsten filament lamps of special design 
were developed which could be electrically modu¬ 
lated with high efficiency at low frequencies (90 
cycles per second) and likewise enabled code sig¬ 
nals to be sent at higher speeds than before, up to 
the limit of the operator at about 30 words per 
minute. 

2. A concentrated-arc projection lamp was de¬ 
veloped which proved successful as a source of high 
intensity and small size. It is especially effective, 
because of its small size, in projecting beams of 
very narrow angles by means of mirrors and lenses. 
This source was used more widely in technical op¬ 
tical applications, such as bore sighting, than in 
direct military infrared equipments for field use. 

3. Microflash lamps were developed in three types, 
one of high intensity with a flash duration of about 
30 psec; a second, also of high intensity, with a flash 
duration of about 3 psec; and a third, especially 
rugged and long-lived but of less peak intensity, 
having a flash duration of about 1 psec, was devel¬ 
oped by the General Electric Company with the 
consultation and advice of the section. Only the 
third type was adopted as an infrared source in 
military equipments; it was employed in a ranging 
apparatus [IRRAD], described later. The second 
type of lamp was employed in some equipment de¬ 
veloped by Section 16.4 for the Ballistics Laboratory 


at Aberdeen Proving Ground to photograph high¬ 
speed shells in flight. 

4. With the advice and aid of the section, the 
Westinghouse Electric Corporation developed an 
electrically modulated cesium-vapor lamp of high 
modulation efficiency at voice frequencies. This 
lamp was developed especially for use in the type E 
communication system, in which the apex of the cone 
of radiation at one-half of the beam-center inten¬ 
sity has the angular dimension of 13 degrees, and 
was in quantity production at the end of the war. 
Other systems providing still wider angles of com¬ 
munication with this lamp were in an advanced 
stage of laboratory development at that time. 

Filters 

Because none of the sources mentioned above 
emit infrared radiation only, but also emit more or 
less visible light as well, it was necessary to develop 
filters to screen out the visible light and transmit 
only the infrared. Since a sharply defined wave¬ 
length limit of visual response does not exist and 
since no filter could be found which cuts off abruptly 
at the “end” of the visual range, a series of filters 
was developed which give varying degrees of visual 
security with a particular source, as demanded by 
the operation in which the equipment is used, while 
maintaining the maximum feasible operating range 
of the equipment. In general, the greater the visual 
security is, the lower becomes the percentage trans¬ 
mission of infrared radiation integrated over the 
range of useful wavelengths for the equipment. 

When the war began, infrared transmitting filters 
of glass were available in several types. Investiga¬ 
tion showed that these could not be materially im¬ 
proved in efficiency of transmission, but consider¬ 
able improvement was made in their ability to 
withstand mechanical and thermal shocks. Glass 
filters of special forms, as in the form of Fresnel 
lenses, were also developed for various military 
beacons. 

Two laboratories worked on the development of 
plastic infrared transmission filters in the hope that 
higher percentage transmissions of infrared radia¬ 
tion could be achieved. Both laboratories were suc¬ 
cessful in doing this; one developed a filter of dyed 
sheets of polyvinyl alcohol sandwiched between 
plane glass plates, and the other a dyed melamine 
formaldehyde plastic, plasticized by an alkyd resin, 



SUMMARY 


3 


which was caused to harden on a glass or trans¬ 
parent plastic supporting plate. Both types of plastic 
filters were made available in a wide variety of 
wavelength versus transmission characteristics and 
both were superior to the glass filters in integrated 
percentage transmission of infrared radiation with 
comparable visual security. They were adopted for 
use in several military equipments and were avail¬ 
able in considerable quantities at the end of the 
war. 

NANCY-TYPE MILITARY EQUIPMENTS 
Glider Position Indicator 

Several military equipments employing near in¬ 
frared radiation were developed to the point of 
demonstrating working laboratory models to the 
Armed Services but were not brought to quantity 
production. One of these was the glider-position in¬ 
dicator [GPI], which had for its purpose the indi¬ 
cation of the position of a glider with reference to 
its tow plane so that the glider pilot could keep the 
glider in its proper position while flying through fog 
so dense the tow plane could not be seen. Infrared 
radiation was prescribed instead of visible light 
only for security reasons, the transmission of near 
infrared radiation through fog being only slightly 
better than that of visible light. The transmitter uti¬ 
lized a tungsten lamp and infrared filter, and a 
thallous sulfide cell was used in the receiver. Satis¬ 
factory operation was demonstrated in flight tests, 
with an estimated range for the laboratory model of 
about 200 feet through the densest clouds likely to 
be encountered. 

Life-Raft Search Equipment 

Another device demonstrated but not put into 
production was designed to locate life rafts or small 
boats adrift at sea. Each such raft or boat would be 
equipped with a set of retrodirective reflectors. A 
search plane would be equipped with a scanning in¬ 
frared transmitter and receiver and would fly over 
the sea and scan a strip of its surface with an in¬ 
frared beam. When some of the radiation from this 
transmitter is reflected back to a tuned receiver on 
the plane, a signal is presented. By means of a ro¬ 
tating shutter the transmitted beam is modulated at 
the frequency of the tuned receiver, and thus false 
signals are avoided. An incandescent tungsten lamp 


is used in the transmitter, and a thallous sulfide 
cell in the receiver. The equipment is not effective 
for search in bright daylight; a night range exceed¬ 
ing 2 miles was demonstrated with the unit land- 
based, and this could probably be met or exceeded 
under favorable weather conditions in an automatic 
airborne model. 

Enemy Infrared Installation Locator (Japir) 

A third device in this class was designed to detect 
the use of and to locate the approximate position of 
enemy near infrared equipments. It consists of a 
near infrared receiver which is to be mounted on a 
search plane. Either unmodulated radiation, voice 
or code modulated signals can be detected. When 
a signal is received the operator knows that a 
source of near infrared radiation is in operation 
and is located in the direction indicated by the re¬ 
ceiver. The range of this device depends upon the 
strength of the infrared source and on the weather 
conditions. As an example, a source consisting of a 
240-watt tungsten lamp enclosed in a red glass 
H globe was detected at a distance of two miles. 

Irrad 

A near infrared detection and ranging system 
constitutes a fourth device not put in quantity pro¬ 
duction but transferred to Navy auspices for fur¬ 
ther specialized development before the end of the 
war. In this system, known as IRRAD, infrared 
techniques are applied to the basic principles uti¬ 
lized in radar. An infrared pulse of about 1 psec, 
emitted from a microflash source, is detected and 
amplified after being reflected from a highly effi¬ 
cient retrodirective reflector target. The signal is 
presented on a cathode-ray tube. A narrow-angle 
transmitted beam is used and is scanned over the 
search area so that a target may be accurately 
located in azimuth and elevation, while the time 
delay between the initial pulse and the received 
signal is accurately translated into target range. 
An electron multiplier with cesium photocathode is 
used to detect and preamplify the signal pulse. A 
single 2-inch target was detected at 4,000 yards 
with the laboratory model; it is estimated that the 
range in clear weather may be extended to 10,000 
yards for an array of six reflectors so oriented as 
to present a uniformly good target around the en- 







4 


SUMMARY 


tire horizon. Diffusely reflecting objects such as 
ships, trees, buildings, etc., can also be detected, 
but only at ranges too short to be of military value. 

Recognition and Code Communication 
System, Type D 

This system was developed for the U. S. Navy as 
an aid in station keeping and communication in con¬ 
voys and task forces. One or two modulated sources 
of near infrared radiation are mounted on board 
each ship in such a way as to cover an angle of 360 
degrees in the horizontal plane and about ±25 
degrees in the vertical plane. One or two receivers 
with amplifiers sharply tuned are mounted on each 
ship on stabilized oscillating stands in such a way 
that two receivers can automatically scan through 
360 degrees in the horizontal plane. A thallous sul¬ 
fide cell mounted at the focus of a parabolic mirror 
can receive radiation in a cone with apex angle of 
11 degrees. The vertical-angle coverage of the 
transmitters is large enough so that they do not 
need to be mounted on stabilized platforms. The 
transmitter can be made to send out a repeated 
identification signal by an automatic keyer, or to 
send code messages up to 30 words per minute, 
limited only by the skill of the operator, by man¬ 
ual operation. The receiver presents the signals by 
means of small flashing lamps, headphones, or a 
loudspeaker. With a 500-watt source in very clear 
weather, a range of about 9 miles has been success¬ 
fully demonstrated; the average clear weather range 
is about 6.5 miles. The visual security distance is 
400 to 1,200 feet, depending on the choice of filter. 
Type D equipment was in quantity production at 
the end of the war. 

A related equipment for plane-to-plane identifi¬ 
cation had also reached the stage of advanced 
laboratory development at this time. 

Voice (and Code) Communication Systems 

At the request of the Bureau of Ships, Section 16.4 
undertook the development of a voice and code com¬ 
munication system over wide-angle beams of near 
infrared radiation. The transmitter of the final 
model, type E, consists of an electrically modulated 
cesium lamp mounted in a parabolic mirror. This 
transmitter produces a conical beam of circular 
cross section, with an apex angle of about 13 de¬ 


grees. Plastic infrared transmission filters are used 
which provide security of 400 yards maximum vis¬ 
ual range. The source and receiver are combined 
into one unit called a transceiver. The receiver con¬ 
sists of a thallous sulfide photocell mounted on the 
axis of a parabolic mirror. Its angle of view is ap¬ 
proximately conical, with apex angle of about 19 
degrees between the points of half-peak response. 
The angles covered by transmitter and receiver are 
wide enough so that it is unnecessary to mount the 
transceiver on a stabilized platform on board ship. 
This equipment has ranges of approximately 6.5 
miles for voice and 9 miles for code in average 
clear weather. At the end of the war, this system 
had just gone into quantity production. 

Similar systems were developed for aircraft, to 
be used either from plane to plane or from plane to 
ground. These differ from the type E system mainly 
in being reduced in size, weight, and power require¬ 
ment to adapt them for aircraft installations. These 
are wide-angle equipments of shorter range than 
type E and are intended primarily for use in com¬ 
munication between planes flying in formation and 
in airborne troop landing operations. 

For plane-to-ground communication, a different 
system, type W, was developed to serve as the 
ground unit in airborne troop landing operations. 
Two of these units also constitute a portable, light¬ 
weight communication system for use between land 
or ship stations. The source consists of a tungsten 
filament lamp mounted in an ellipsoidal reflector. 
The radiation is mechanically modulated by a vi¬ 
brating mirror. The receiver uses a thallous sulfide 
cell and is patterned after the receiver of the type E 
system. The equipment is portable and can be oper¬ 
ated from a small portable storage battery. The 
entire equipment weighs only 18 pounds and can 
be carried down from a plane by a paratrooper. 
The range for a pair of these units is about 1.5 to 
3 miles in average clear weather, depending on the 
angular width selected for the transmitted beam. 
Quantity procurement was being planned at the 
end of the war but had not been actually initiated. 

Two other voice communication equipments were 
developed and demonstrated but were not put into 
quantity production. One of these employs plane 
polarized infrared radiation which passes through 
a photoelastic shutter and is modulated and trans¬ 
formed into circularly polarized radiation. Only a 
receiver having a properly oriented polarizing screen 



SUMMARY 


5 


can receive the signals, although the apparently un¬ 
modulated radiation may be detected, for example, 
by an infrared telescope. The final laboratory model 
of this equipment has a range of about 4 miles in 
average clear weather and provides very high secu¬ 
rity against interception of messages. The device is 
necessarily more complicated and less efficient than 
other systems described here. 

• The other equipment was originally developed in 
France and was reconstructed here by a French 
naval officer. Its receiver is conventional, but the 
transmitter employs a special source consisting of 
a glow discharge lamp mounted at the focus of a 
parabolic reflector. This lamp has high modulating 
efficiency at frequencies of 100 or 200 kilocycles, 
and so a high-frequency carrier wave modulated by 
the voice is employed. Such a system can be con¬ 
structed so that the message cannot be received ex¬ 
cept by a receiver tuned to the frequency of the 
carrier wave. In this way greater security against 
interception of messages can be attained and several 
different frequency channels of communication can 
be provided. 

FAR INFRARED DEVELOPMENTS 

Since the far infrared equipments of current mil¬ 
itary interest do not require auxiliary sources or 
filters, fundamental development of components was 
limited to new or improved detectors and optical 
components for receiving and utilizing self-emitted 
thermal radiations. 

Improved methods were developed for producing 
various types of thermal detectors having faster 
response and lower threshold detectable energy lim¬ 
its than before the war. These include thermopiles, 
evaporated metal-strip bolometers, and thermistor 
bolometers of various types. The thermistor bolom¬ 
eters experienced the greatest improvement and 
were most widely used in the military equipments 
developed by this and other NDRC sections. In 
addition, a considerable improvement in the per¬ 
formance of equipments resulted from fundamental 
studies on scanning systems and optical compo¬ 
nents for infrared receivers, including mirrors, 
Schmidt plates, transmitting cover or window ma¬ 
terials, and protective coatings. 

One contract of the section also made extensive 
studies, in cooperation with the Armed Services, on 
the comparative characteristics of many different 


detectors and receivers, both domestic and foreign. 
Consultation, evaluation, and assistance to military 
agencies and other Office of Scientific Research and 
Development [OSRD] divisions on matters pertain¬ 
ing to far infrared components and techniques con¬ 
stituted an important phase of the section’s program. 

CASPAR-TYPE MILITARY EQUIPMENTS 

All of the far infrared equipments developed em¬ 
ploy “Caspar” technique, that is, they operate on 
radiation emitted by the target acting as a self- 
emitting infrared source. Two of these equipments 
were designed especially to detect ships, the port¬ 
able ship detector [PSD] and the stabilized ship 
detector [SSD]. 

The PSD consists of a receiver head mounted on 
a tripod and appropriate power supplies, amplifiers, 
and signal indicator. The receiver head oscillates 
so as to scan periodically through a small angle 
along the horizon. When a ship lies within the angle 
scanned, an aural signal is given. The head may be 
turned by hand so as to scan a wide area of the 
sea in successive steps. This equipment has a range 
in average clear weather of 4,000 to 10,000 yards, 
depending on the size of the target ship. It was 
superseded by the SSD. 

The SSD was developed to be mounted on a stabi¬ 
lizing platform. It detects the presence of a ship 
within its range and gives the bearing with an error 
of less than ^4 degree. The head oscillates through 
an angle which can be quickly set or changed from 
a few degrees up to 180 degrees. Radiation from 
the target ship or ships falling first on one and then 
on the other of a pair of thermistor strip bolometers 
produces signals which are recorded on a chart in 
such a way that the bearings of all the ships in 
the field of the receiver can be quickly read. Rela¬ 
tive movements of the target ships with respect to 
the ship bearing the SSD can be followed from the 
record on the chart. The SSD has a range in average 
clear weather varying from 4,000 to 25,000 yards, 
depending upon the size of the target ship. A quan¬ 
tity-production program was being set up when the 
war ended. 

The SSD lends itself advantageously to operation 
in combination with other equipments. A combina¬ 
tion of the SSD with a radar set was planned, but 
the end of the war terminated its development be¬ 
fore completion. The idea was to detect the target 



6 


SUMMARY 


ship and determine its bearing with the SSD in com¬ 
plete secrecy, while maintaining radar silence, and 
then to determine its range by a brief operation of 
the radar unit. Another possible combination con¬ 
sists of the SSD and a narrow-angle voice or code 
communication system. In this combination the 
SSD would be used to train and direct the beams of 
the communication system. These would train simul¬ 
taneously so that when the SSD detected a friendly 
ship the beam of the communication equipment 
would face upon it, and so contact could be im¬ 
mediately established and maintained through an 
automatic lock-on system without searching with 
the transmitted beam of the communication sys¬ 
tem. This arrangement would permit the use of 
very narrow-angle beams and would thus greatly 
increase the security of communications. 

The portable NAN detector [PND] was devel¬ 
oped originally to detect personnel but was found 
applicable for the detection of tanks, motor vehicles, 
ships, small boats, and buildings as well. In its 
simplest form it is a completely self-contained unit 
operating from dry batteries. This model weighs 
less than 10 pounds exclusive of mounting tripod 
and has a range in average clear weather of 500 
yards for a man, about 2,500 yards for a small tank 
and several thousand yards for a ship. The detector 
is a pair of strip thermistor bolometers mounted in 
a cell with a rock salt or silver chloride window at 
the focus of a 7-inch parabolic mirror. The signal 
is presented by means of headphones. A production 
model called Penrod was developed but a very lim¬ 
ited number had been made by the end of the war. 
One laboratory model PND was used to guard the 
crossing of a river in Germany. It was set up to 
give warning in case any person or object crossed 
through its cone of observation. Similar uses for this 
device were proposed, such as to give warning of the 
approach of enemy troops or tanks along a road or 
passageway within the cone of observation of the 
PND and to warn of enemy troops coming in to¬ 
ward shore on a beachhead at night. 

Outgrowths of the PND were the scanning NAN 
detector [SND] and the thermal map recorder 
[TMR]. The SND has a scanning head which 
causes images of objects in the field of view to sweep 
across first one then another thermistor bolometer. 
The signals are recorded on a chart as small vertical 
lines. The thermal map recorder is similar to the 
SND in general features but is a more refined in¬ 


strument and presents a better record, like that of 
the SSD, consisting of small dots at points along a 
line corresponding to the relative positions of the 
targets. The chart advances a small amount between 
two sweeps of the scanning head so that the signals 
from a given target at rest form a dotted line along 
the length of the record strip. Motion of a target is 
indicated by a slanting or curved dotted line formed 
by its signals. 

These instruments construct a thermal map of 
the area scanned by them if they are carried for¬ 
ward by a plane. Any object differing in indicated 
temperature from its surroundings gives a signal 
and thus buildings, trees, vehicles, roads, creeks, 
and banks of rivers or shore lines are charted. Some 
of these give stronger signals than others and the 
gain of the amplifier can be set so as to eliminate 
the weaker signals. Thus it is possible under favor¬ 
able conditions to record signals from tanks on a 
road and not the road or trees along side it. Because 
of the unsteady motion of a plane, it was found 
necessary to mount the instrument on a stabilized 
platform in order to get a consistent thermal map. 
Plans were under consideration to do this when the 
war ended. The ranges of the SND and of the TMR 
are about the same as those of the PND. The war 
ended before quantity production of these instru¬ 
ments was achieved. 

Another equipment under development for the 
Bureau of Aeronautics, USN, is known as type L. 
It was designed to be used on a drone and was in¬ 
tended to send signals by means of a radio link to 
the plane controlling the flight of the drone. The 
signals were to indicate the angle between the line 
of flight of the drone and a line from the drone to 
the target and thus permit the operator in the con¬ 
trolling plane to direct the drone in a collision 
course. When the war ended the development had 
reached the stage of demonstrating the detection of 
a target ship and presenting its signals on a cathode- 
ray oscilloscope while the equipment was borne by 
an airplane. 

Other Developments 

Space does not permit discussion of other appli¬ 
cations of bolometers with which Section 16.4 as¬ 
sisted by advice and counsel and by making avail¬ 
able developments on bolometers carried out under 
its auspices. These applications are reported else- 





SUMMARY 


7 


where and can be only briefly enumerated here. 
Bolometers were applied, for example, in heat¬ 
homing bombs, bombsight with angular rate release, 
and airplane detecting equipments developed under 
the auspices of other sections of NDRC. 

DEVELOPMENTS IN THE INTERMEDIATE 
INFRARED REGION 

Lead sulfide cells had been developed to the point 
at which they were about ready for quantity pro¬ 
duction when the war ended. Two types were de¬ 
veloped, one operating at ambient temperatures and 
the other at reduced temperatures resulting from 
cooling with solid C0 2 (dry ice). A reservoir was 
provided which would hold enough dry ice to permit 
operation for several hours without refilling. These 
cells are sensitive to longer wavelength radiation 
than are thallous sulfide cells and have better fre¬ 


quency response characteristics. It is likely that 
they would have been widely adopted to provide 
additional communication channels at wavelengths 
greater than 1.5 p had the war continued another 
year. Because of their sensitivity to longer wave¬ 
lengths, filters providing greater visual security for 
a given source could be used with lead sulfide cells 
than with thallous sulfide cells. Tests on other types 
of photoconductive materials were in progress when 
the war ended. 

Since the lead sulfide cell is sensitive to infrared 
radiation of wavelengths up to about 3.5 p, it can 
be used to detect objects at prevailing outdoor tem¬ 
peratures. Hence, Caspar-type receivers with lead 
sulfide cells are also possible. One such was con¬ 
structed and some tests were made on the detection 
of ships. The ranges were less than those of the in¬ 
struments using bolometers, but considerable im¬ 
provement may be anticipated from further work. 












































































































































































































:* 




















# 






Chapter 1 

NEAR INFRARED SOURCES 

By Winston L. Hole 


11 INTRODUCTION 

T he purpose of this chapter is to describe the 
nature and characteristics of the near infrared 
[NIR] sources which have been found most suitable 
for use in non-image-forming applications, includ¬ 
ing specifically equipments and military systems 
which will be described in greater detail in Chapters 
4, 5, 6, and 7. It includes a description of certain 
types of the incandescent tungsten lamps in new or 
improved adaptations and, in addition, a descrip¬ 
tion of several other sources newly developed under 
NDRC or Navy auspices which contributed to the 
NDRC near infrared program. 

111 Types of Sources 

The near infrared sources described in this chap¬ 
ter fall within two main categories, namely, incan¬ 
descent tungsten lamps and special gaseous dis¬ 
charge lamps. The treatment of standard incan¬ 
descent tungsten lamp types listed in the bulletins 
of commercial manufacturers is limited principally 
to an enumeration of their physical characteristics 
and a brief description of the radiation character¬ 
istics of the beacons or transmitters in which they 
have been used. One special incandescent lamp de¬ 
veloped by the General Electric Company at the 
request of NDRC Section 16.4 and one rather novel 
adaptation in the use of an already standardized 
type of lamp are described in somewhat greater de¬ 
tail in Section 1.2.2. The construction, mode of 
operation, and general radiation characteristics of 
the special gaseous discharge lamps developed under 
the auspices of this section are treated still more 
fully, and a similar treatment, except for the details 
of construction, is given for the cesium vapor lamp, 
developed under a Navy (BuShips) contract with 
the Westinghouse Electric Corporation. This lamp 
was extensively used in the military equipment de¬ 
veloped by one contract of this section. 


Types of Applications 

The near infrared sources described in this chap¬ 
ter have been used in conjunction with appropriate 
optical systems, infrared transmitting filters, and 
radiation detectors in the development of military 
equipments under contracts administered by NDRC 
Section 16.4. The various applications include voice- 
code communication systems, recognition and code 
communication systems, a position-indicating sys¬ 
tem for gliders, a system for the detection of retro- 
directive triple-mirror reflectors, and systems for 
the detection and ranging either of retrodirective 
triple-mirror reflectors or of large, diffusely reflect¬ 
ing targets such as ships. Another important appli¬ 
cation primarily involving visible rather than infra¬ 
red radiation lies in the ballistic photography of 
high-speed projectiles with a gas-filled, high-inten¬ 
sity flash lamp especially developed to provide a 
flash duration short enough for this purpose. A de¬ 
scription of other components and of the complete 
systems is given in subsequent chapters. More de¬ 
tailed information on each subject is given in the 
Bibliography. 

113 Nomenclature System for Near 
Infrared Photometry 

The free interchange of ideas and comparison of 
results between various laboratories concerned with 
near infrared developments was at first hampered 
by the lack of an adequate system of nomenclature 
and units for near infrared photometry. The early 
recognition of the need in this field for a uniform 
basis of testing and comparison led to the proposal 
of a system which was subsequently approved and 
adopted for official use by the Combined Communi¬ 
cations Board of the Combined Chiefs of Staff. The 
officially approved report 1 in which this system is 
presented in schematic version is reproduced in the 
Appendix. Its terminology is generally followed 




9 



10 


NEAR INFRARED SOURCES 


throughout the near infrared portions of the volume 
whenever the contribution of wavelengths beyond 
the visible must be taken into account or when 
a radiation detector having spectral response 
characteristics different from those of the eye is 
referred to. 

This system, sometimes referred to as the hololu- 
men system, has the advantages of being very 
closely associated with an established system of 
visual photometry and of using the total radiation 
from the universally available incandescent tung¬ 
sten lamp, properly standardized at the color tem¬ 
perature of 2848 K, as a standard of comparison 
for taking cognizance of the near infrared radiation 
component from this and other types of source with 
respect to any detector of visible and near infrared 
radiation. 

Although in practice certain difficulties and objec¬ 
tions to the system have arisen, its creation and 
adoption have been amply justified in terms of 
scientific as well as purely practical results. It has 
helped to eliminate some of the approximations and 
guesswork which, in the absence of such a system, 
previously existed in the field of near infrared pho¬ 
tometry. It further constitutes a valuable step to¬ 
ward a badly needed fundamental system of meas¬ 
urement and nomenclature for general use in 
connection with selective radiation sources, filters, 
and detectors. 

An outstanding example of its potential value was 
demonstrated by four cooperating laboratories under 
the sponsorship of NDRC Section 16.4. Each lab¬ 
oratory was provided with a set of primary stand¬ 
ards consisting of incandescent tungsten lamps, 
infrared transmitting filters, and cesium-surface 
vacuum phototubes. The color temperature and in¬ 
tensity of the lamps, the effective holotransmission 
[ehT] values of the filters, and the spectral response 
characteristics and responsivities of the phototubes 
were initially calibrated and intercompared at a 
single laboratory. On the basis of these results, and 
through preliminary selection of closely matched 
units, correction factors not exceeding 10 per cent 
were determined for each combination of source, 
filter, and detector within the group of primary 
standards assigned to each laboratory. Subsequently 
it became possible for the first time for one labora¬ 
tory to duplicate closely the results of another in 
measuring the characteristics of a given source, with 
a considerable saving in the time and effort pre¬ 


viously devoted to the explanation or elimination of 
conflicting results. 

It is assumed that the reader is familiar with the 
basic principles of optics and photometry and with 
the fundamental vocabulary used by textbooks in 
these fields. More specialized terminology which 
has been found of sufficient value to warrant its 
adoption within NDRC groups will be defined at 
appropriate points in this and subsequent chapters. 

1,1,4 Methods of Modulation; 

Modulation Ratio 

The successful operation of a non-image-forming 
system frequently requires that the source be modu¬ 
lated or pulsed in such a way that the radiation 
emitted by it varies in a regular or periodic manner. 
In addition it may be necessary to interrupt the 
modulated radiation, as for code transmission, or to 
superimpose additional modulation at a substan¬ 
tially different frequency, as for modulated carrier- 
wave communication systems. Details of these re¬ 
quirements for certain applications, and of the 
methods developed to meet them, are given in Chap¬ 
ters 4, 5, 6, and 7. 

All of the sources described in this chapter are 
electrically operated, but not all of them can be suc¬ 
cessfully modulated at the source by electrical 
means to meet the frequency, waveform, or other 
requirements of a specific application. Incandescent 
tungsten lamps, for example, cannot be efficiently 
modulated even at the middle audio frequencies by 
direct operation from an a-c source. Every source 
has certain limitations in this respect. Each must, 
therefore, be considered separately, (1) as a source 
of steady radiation which may be modulated by 
mechanical means after it leaves the source, and 
(2) as a source of radiation which may be modu¬ 
lated or pulsed directly by the electric power supply 
from which it is operated. For example, there may 
be a considerable time lag between the operating 
current and the corresponding phase of the emitted 
radiation, as well as a considerable difference be¬ 
tween the depth of modulation of the operating 
current and the resulting radiation. In addition, all 
wavelengths emitted by a source do not have the 
same modulation characteristics under identical con¬ 
ditions of operation. 

It is evident that a great many details must be 
given careful consideration in choosing the source 


AIju J. 




INCANDESCENT TUNGSTEN LAMPS 


11 


of highest overall efficiency for a particular appli¬ 
cation. No attempt will be made here to analyze or 
enumerate all of these factors in relation to wave¬ 
form, chopper efficiency, power consumption, etc. 
However, in considering electrically modulated 
sources, three basic definitions for the case of simple 
sine wave modulation will be of value. 

By “per cent current modulation” is meant 
Amplitude of the sine wave modulating 

_ current _ - _ _ x ioo 

Direct-current operating value without 
modulation 

By “per cent modulation of emitted radiation” is 

meant 

Amplitude of the modulated component of 

_ radiant intensity _ ^ 

Radiant intensity during operation on 
unmodulated direct current 

By “modulation ratio” is meant 

Per cent modulation of emitted radiation ^ 
Per cent current modulation used for 
operating the source 

Thus a modulation ratio of unity is achieved only 
if the depth of modulation of the emitted radiation 
is equal to that of the modulating current. The val¬ 
ues of the modulation ratio which may be achieved 
with a given source are a significant measure of its 
effectiveness as an electrically modulated radiation 
source. 

12 INCANDESCENT TUNGSTEN LAMPS 

Because the wavelength of peak spectral radiant 
intensity lies near one micron for incandescent 
lamps at about 3000 K, these lamps constitute a 
valuable and widely used source for near infrared 
radiation. Their design and detailed characteristics 
are discussed more fully in the Summary Technical 
Report of Division 16, Volume 4, Chapter 5. Only 
those tungsten lamps specifically used in the mili¬ 
tary equipments developed by Section 16.4 are enu¬ 
merated in the present chapter, together with a brief 
description of their mode of application. Other lamps 
have, of course, been widely used in the laboratories 
and in test apparatus auxiliary to the equipments 
described below. The lamps are identified by means 
of the standard abbreviations used in the technical 


bulletins and catalogs issued by the commercial 
lamp manufacturers. 

Lamps of Standard Type for 
Miscellaneous Purposes 

Communication System with Photoelastic 
Shutter 2>3 

A voice communication system utilizing a photo- 
elastic shutter was developed under Contract 
OEMsr-576 with the Massachusetts Institute of 
Technology. The shutter consists of a clear glass 
plate in which a standing elastic wave of constant 
frequency but variable amplitude is excited by means 
of one or more quartz crystals electrically driven 
by a low-power radio transmitter. Plane-polarized 
radiation impinging on the shutter is rendered ellip- 
tically polarized by the photoelastic birefringence 
resulting from the elastic stresses set up in the shut¬ 
ter. A properly oriented analyzing sheet of infrared 
polarizing material is needed over the radiation de¬ 
tector to permit reception of the transmitted mes¬ 
sage. Since, in addition, the intelligence is trans¬ 
mitted only within a rather narrow cone of invisible 
radiation, the transmitter possesses an exceptionally 
high degree of security. The infrared polarizing 
sheets used in both transmitter and receiver were 
produced by the Polaroid Corporation. The com¬ 
plete communication system and its operating char¬ 
acteristics are described in Chapter 4. 

In the first laboratory model, a 420-watt Mazda 
incandescent aircraft landing lamp (G-25 bulb, C-2 
filament, 12 volts, 35 amperes) rated at 10,500 ap¬ 
proximate initial lumens was used as the source. It 
was operated from an a-c line through Variac and 
transformer controls. Radiation from the filament 
was focused by an 8-inch diameter, semiprecision 
parabolic reflector within an area about 1.5 inches 
square about 20 inches from the source. At this 
point the radiation passed successively through an 
infrared polarizing sheet, a Polaroid Corporation 
infrared transmitting filter, and the photoelastic 
shutter, each component being 2 inches square. The 
emerging radiation was then projected by a clear 
glass lens of 8-inch diameter and 12-inch focal 
length in a cone subtending approximately 20 de¬ 
grees. Even when the shutter was operating, the 
transmitter was completely invisible to the dark- 
adapted eye at any distance. 


fTPwnirTF.n 
i\£jo i iiroi'Bu 






12 


NEAR INFRARED SOURCES 


In later models the size of the shutter was suc¬ 
cessfully increased to 4, 6, or 8 inches square and 
at the same time provision was made for using a 
more intense source, such as a sealed-beam type 
tungsten lamp. A wide variety of adjustments in 
transmitter beam widths, power consumption, etc., 
thus became possible by correlating the choice of a 
particular source lamp with other optical compo¬ 
nents selected for the transmitter. With high-power 
sources it was necessary to include a water filter or 
a forced-air cooling system to prevent the thermal 
destruction of the plastic membrane, infrared polar¬ 
izing and filter sheets. 

A Navy (BuShips) contract for the construction 
of one or more improved communication systems of 
this type was later negotiated with White Research 
Associates, Cambridge, Massachusetts. A final re¬ 
port on this development was not available at the 
date of this writing. 

An Infrared Glider Position Indicator 4 

Equipment for indicating the position of a glider 
with respect to its tow plane was developed by the 
University of Michigan under Contract NDCrc-185. 
A lamp of the photocell-exciter type (T-8 bulb, C-6 
filament, 10 volts, 5 amperes, single-contact bayonet 
base) was used as the radiation source in this equip¬ 
ment. An ammeter and a rheostat control were 
included in the lamp circuit which was operated 
from the 12-volt battery of the tow plane. 

The center of the lamp filament was located at 
the focal point of a Bart parabolic mirror of 2-inch 
focal length and 3-inch diameter. The collimated 
beam reflected from the parabolic mirror was then 
reflected successively from two plane mirrors (front- 
surface-aluminized glass), each of which was vi¬ 
brated about one of a pair of mutually perpendicu¬ 
lar axes in such a way that the projected beam 
described a Lissajous pattern in space. The trans¬ 
mitter assembly was covered by a clear glass win¬ 
dow to which was attached a sheet of Polaroid 
XR7X25 infrared transmitting filter. 

The angle subtended by the projected image of the 
lamp filament was approximately one degree hori¬ 
zontally by five degrees vertically. By means of the 
two vibrating mirrors this image was scanned in 
such a manner as to give fairly uniform coverage 
over a space field subtending about 20x20 de¬ 
grees at a repetition frequency of 14 per second. 
The characteristics of the scanning pattern were 
fixed by the choice of a 6 to 1 ratio for the fre¬ 


quencies of the two vibrating plane mirrors. The 
complete system, described more fully in Chapter 
7, constituted a rudimentary infrared television 
system with the radiation pickup and presentation 
unit located in the glider within the television scene. 
Because of the space modulation (or scanning) sys¬ 
tem in which this source was used, and the time- 
dependent response characteristics of the thallium 
sulfide cell used as the radiation detector, informa¬ 
tion on the beam candlepower and similar charac¬ 
teristics for this source is not easily related to the 
operating characteristics of the system and is there¬ 
fore not given here. 

Retrodirective Reflector Target Locator 5 

An apparatus for detecting and locating retro- 
directive reflectors by means of infrared radiation 
was constructed by the University of Michigan 
under Contract NDCrc-185. The radiation source 
was a 300-watt projector-type tungsten lamp (T-10 
bulb, C-13 filament, 30 volts, 10 amperes, medium 
prefocus base) operated from a 24-volt d-c aircraft 
power supply with a control rheostat. If needed, a 
6-volt storage battery could be used to boost the 
line voltage up to the normal rating for this lamp. 
A 3-inch, second-surface glass spherical mirror of 
commercial grade was located on one side of the 
lamp with its center of curvature at the center of 
the filament. On the other side of the lamp was a 
Fresnel-type optical signal lens of 5%-inch diam¬ 
eter and 314 -inch focal length, located with its focal 
plane at the lamp filament. An image of the filament 
region, strengthened by the spherical backing mir¬ 
ror and fairly uniform over an approximately square 
zone subtending a total angular width of about 4 
degrees, was projected with this arrangement. The 
beam candlepower with no filter over the source 
was approximately 125,000. The angular dimensions 
of the beam could be increased to approximately 10 
degrees square, with a corresponding decrease in 
beam candlepower by a factor of 6 to 7, by inserting 
a “beam spreader” of hammered glass treated with 
clear lacquer. An infrared transmitting filter con¬ 
sisting of two sheets of Polaroid film cemented to 
a clear glass plate, having a transmission approxi¬ 
mately equal to that of XR7X25 material, could be 
inserted in the beam to provide a high degree of 
visual security. 

Because of the highly accurate retrodirective re¬ 
flecting property of the triple mirrors which were 
used as targets, the source and the coaxial cylin- 






INCANDESCENT TUNGSTEN LAMPS 


13 


drical shutter by which the radiation was modulated 
were designed as compactly as possible and mounted 
on the axis of the 12-inch reflector which focused 
the signal flux on the radiation-sensitive element of 
the receiver. In this respect the equipment was sim¬ 
ilar to the IRRAD equipment, described in Chapter 
6 , for detecting and ranging the same type of target. 

The cylindrical modulating shutter was driven 
by a 24-volt d-c governor-controlled motor and pro¬ 
duced trapeziform time modulation of the projected 
flux at 90 cycles. To this shutter was attached a 
small blower which provided forced air ventilation 
of the lamp. A separate blower without a shutter 
could be used interchangeably with the preceding 
combination when it was desired that the signal 
flux be modulated by a chopper at the triple-mirror 
target in order to minimize the effects of atmos¬ 
pheric backscatter. The entire equipment and its 
performance characteristics are described in Chap¬ 
ter 5. 

Portable Hand-Held Infrared Optical 
Telephone (Type W) c 

A lightweight infrared voice transmitter-receiver 
development begun by the University of California 
under Contract OEMsr-1073 with the direction of 
Section 16.5 was later transferred to Section 16.4 
and successfully completed under these auspices. 
In the early models a 40-watt, 50-candlepower auto¬ 
mobile spotlight bulb was used. A 10-pound, 6-volt 
portable storage battery would permit nearly two 
hours of continuous operation, or, if available, an 
automobile or other large storage battery would per¬ 
mit longer operation without recharging the battery. 
With this source the angular dimensions of the 
transmitted beam were approximately 2x3 degrees. 
In the final models a 4x5-degree beam was obtained 
through the use of a 100-watt incandescent tung¬ 
sten source (5.5 volts, 18 amperes, average life only 
5 to 10 hours because of operation at about 3400 K) 
specially constructed for this purpose by the Gen¬ 
eral Electric Company. The period of continuous 
operation from the portable storage battery is cor¬ 
respondingly reduced to about one hour. A Lucite 
beam spreader attachment, which widens the 
transmitted beam to approximately 10x12 degrees, 
could be inserted when desired. When no filter is 
included in the transmitter, the beam candlepower 
is 7,000 to 8,000 for the 4x5-degree beam. But the 
Lucite beam spreader lowers this to 1,200 to 1,600. 

The transmitter is given a high degree of visual 


security by the insertion of a suitable infrared filter 
adjacent to its external protecting window. The 
optimum filter for a particular situation depends 
upon the beam candlepower being used. Suitable 
filters were supplied by both the Polaroid Corpo¬ 
ration under Contract OEMsr-1085, and Ohio 
State University under Contract OEMsr-987. The 
filters for operational tests were chosen so as to limit 
the range of visibility for a dark-adapted naked eye 
to approximately 2 per cent of the operating range 
for any given condition. For the 4x5-degree beam 
this distance is about 100 yards for an observer in 
the center of the beam, and the filter used corre¬ 
sponds approximately to the Polaroid XR7X25 
material. Less dense filters may be successfully 
used with the larger beam angles. 

Optical Pi'inciples of Transmitter. The trans¬ 
mitter contains three mirrors: (1) an ellipsoidal 
mirror 4.5 inches in diameter; (2) a concave glass 
grid mirror 4.25 inches in diameter, coated with 
reflecting strips of aluminum or gold approximately 
y 1Q inch wide, alternated with clear spaces of ex¬ 
actly equal width; (3) an electrically vibrated con¬ 
cave mirror in the shape of an oval % 6 x% inch, lo¬ 
cated at the focus of the grid mirror. Radiation from 
the lamp filament is reflected from the ellipsoidal 
mirror through the clear spaces of the grid mirror 
and is focused on the surface of the concave mirror. 
The concave mirror focuses an image of the grid 
back on the surface of the grid mirror. The diffuse 
image of the filament on the surface of the vibrating 
mirror acts as a source of variable intensity for the 
transmitter beam, which is projected by the grid 
mirror. The intensity of the transmitted beam is 
varied in the amount of radiation reflected from the 
grid mirror as the image of the grid is oscillated 
across the mirror surface by the motion of the vi¬ 
brated concave mirror. 

Other Infrared Optical Telephones. Incandescent 
tungsten lamps have been used as the radiation 
source in several German, Italian, and Japanese 
optical communication systems for voice and code, 
some of which have bden analyzed in reports 7 ’ 8 > 9 
submitted by the University of Michigan and 
Northwestern University. The more successful sys¬ 
tems, though differing in detail, utilize some me¬ 
chanical-optical principle for modulating the steady 
radiation from a direct-current-operated source. 
Systems in which the radiation is modulated by 
operating the source on voice-modulated current 
have apparently been a good deal less successful. 






14 


NEAR INFRARED SOURCES 


Some of these communication systems are briefly 
described, together with their operating character¬ 
istics in Chapter 4. 

Microflux Source for a Photocell Test Set 10 

In order to permit standardization and easy inter¬ 
comparison of test results obtained by various con¬ 
tracts of Section 16.4 and by the associated military 
laboratories in which related work was being carried 
out, a photocell test set was designed and con¬ 
structed by the University of Michigan under Con¬ 
tract NDCrc-185. The equipment consisted of a 
microflux source together with especially designed 
electrical measuring equipment. In accordance with 
the system mentioned in Section 1.1.3, a tungsten 
lamp calibrated for operation at the color tempera¬ 
ture of 2848 K is used as the source of radiation, 
attenuated in the microflux source to a known mag¬ 
nitude of the order of a few microhololumens, for 
testing the responsivity, frequency response charac¬ 
teristics, and noise holothreshold of near infrared 
radiation detectors. 

A recording microphotometer type of lamp (for 
example, Mazda 1708 with C-6 filament, S—11 bulb, 
single-contact bayonet base, rated at 4.8 volts, 4.5 
amperes) proved to be most convenient for this 
purpose. As mounted in the test set, the holocandle- 
power of this lamp is approximately 20 in a direction 
perpendicular to the filament when the lamp is 
operated at the color temperature of 2848 K. A num¬ 
ber of other small tungsten lamps, for example, 
lamps of the photocell exciter type having a C-6 
filament, are also suitable. Since at the mentioned 
color temperature most of these lamps are operated 
at 15 to 20 per cent below their rated voltage, their 
life is quite long. It is found that, after being prop¬ 
erly seasoned, a lamp may be used for a period of 
100 hours or more without being recalibrated, while 
an accuracy of ±5 per cent or better is maintained 
which is entirely satisfactory for ordinary test 
purposes. This equipment and its mode of use are 
more fully described in Chapter 3. 

1,2,2 Recognition and Code Communication 
Sources 

One of the more important applications of incan¬ 
descent tungsten lamps is in the beacons which con¬ 
stitute a part of the type D recognition and code 
communication systems described more fully in 


Chapter 5. These systems were developed under a 
BuShips project. A closely related development for 
BuAer is briefly described at the end of this 
section. 

The Type D Beacon 

The first type D equipment 11 constructed and 
demonstrated to the Armed Services was intended 
for ship-to-plane recognition only, without provi¬ 
sion for communication between two stations. A 
standard tungsten projection lamp (C-13 monoplane 
filament, T-20 bulb, 115 volts, 500 watts, medium 
prefocus base) was used as the source of radiation. 
The rated life for this lamp is 50 hours, and the ap¬ 
proximate initial lumens, 13,000. Because of the 
large thermal inertia, this lamp could be operated 
satisfactorily from a standard 60-cycle line, since 
the receiver was so constructed that the small 
amount of 120-cycle modulation did not prevent 
satisfactory operation. 

The lamp was surrounded by two coaxial cylin¬ 
drical brass shutters to modulate and code the 
emitted radiation. The inner modulating cylinder 
contained nine segments of 20 degrees each, sepa¬ 
rated by clear segments of equal width. The outer 
cylinder was made in adjustable sections such that 
any code letter of three elements could be super¬ 
imposed on the radiation already modulated by the 
inner cylinder. The repetition rate of the code rec¬ 
ognition symbol was 40 times per minute. The tops 
as well as the sides of the cylinders were segmented 
and the base of each and the mounting for the bea¬ 
con were considerably below the lamp filament so 
that the vertical zone of coverage extended from 
below the horizontal plane to about 10 degrees from 
the zenith. Azimuthal coverage of 360 degrees was 
provided at all angles of elevation within this ver¬ 
tical zone of irradiation. 

The rotating cylinders were enclosed in a glass 
dome 5 inches in diameter and 9 inches high, the 
inner surface of which was covered with a layer of 
Polaroid infrared transmitting filter approximately 
equivalent to XR7X25. With this filter the visible 
range of the source for the dark-adapted naked eye 
was only a few feet, while the magnitude of the 
receiver response using a thallium sulfide cell de¬ 
tector (see Chapter 3) was reduced to about 30 per 
cent of its value with no filter. To prevent overheat¬ 
ing of any portion of the beacon, the air inside the 
dome was circulated by means of a small blower. 





INCANDESCENT TUNGSTEN LAMPS 


15 


In the construction of subsequent models, 12 em¬ 
phasis was placed on providing for communication 
as well as recognition, primarily from ship to ship 
rather than from ship to plane. Means for communi¬ 
cation were provided through a manually operated 
key which controlled the application of power to the 
lamp filaments. The lamp could also be operated 
from an automatic coder designed to transmit con¬ 
tinuously the code letter or group of letters desig¬ 
nated as the recognition symbol for the given 
period. The outer of the two rotating cylindrical 
shutters in the model described above was therefore 
dispensed with, since its function of coding the 
transmitted radiation was superseded in this man¬ 
ner, and the modulating shutter was redesigned. 

At the request of NDRC a special tungsten lamp 
was developed by the Lamp Development Labora¬ 
tory of the General Electric Company for use in this 
beacon. The new lamp was designed to permit higher 
keying speeds and more uniform intensity distribu¬ 
tion in the plane perpendicular to the axis than 
were possible with the standard projection lamp used 
in the earlier model. A lamp of the type finally 
developed for Navy use is shown in Figure 1. It has 
a T-14 heat-resistant envelope and medium bipost 
base with a maximum overall length of 6% inches. 
The 4C12 filament is rated at 110 volts, 500 watts, 
with an approximate life of 200 hours. Its mean 
horizontal candlepower, when mounted base down, 
is approximately 1,000. 

The lamp is enclosed in a concentric cylindrical 
brass modulating shutter having three open and 
three closed segments, each 60 degrees in width. A 
synchronous motor drives the shutter at 30 rps, 
thereby modulating the emitted radiation at 90 
cycles per second. Around the shutter is a small 
inner cylinder of clear glass, and around this is a 
cylindrical Fresnel marine lens. On the outer sur¬ 
face of the cylinder and the inner surface of the 
Fresnel lens is a sheet of Polaroid infrared trans¬ 
mitting filter selected so that the combined trans¬ 
mission by the two layers is approximately equiva¬ 
lent to that for XR7X25 material. Thus, if one layer 
of the filter material fails or is fractured, the visual 
security of the beacon is not impaired to the same 
extent as if there were only one layer. To insure 
360-degree azimuthal coverage it was necessary to 
install two beacons on each ship to avoid obscura¬ 
tion of certain zones by the ship’s own structure. The 
Fresnel lens concentrates from one to three times the 



Figure 1 . Incandescent tungsten lamp for the type 
D beacon. 

intensity which would exist in its absence into a zone 
extending ±11 degrees from the horizontal, while 
at larger angles the intensity is correspondingly re¬ 
duced. Use of the lens is therefore advantageous 
only for signaling near the horizontal plane, as from 
ship to ship, and is to be avoided when nearly 






















16 


NEAR INFRARED SOURCES 


hemispherical coverage is desired, as from ship to 
plane. In later models an infrared transmitting glass 
was used either for the plain inner cylinder or for 
the Fresnel lens. Forced air circulation and conduct¬ 
ing fins were incorporated in the beacon to provide 
adequate heat dissipation during continuous use at 
high ambient temperatures. 

With these transmitters and the associated re¬ 
ceivers it is possible to transmit code at an esti¬ 
mated speed of 10 to 12 words per minute. One 
source of difficulty initially encountered in code re¬ 
ception arose from the fact that the lamp filament 
cooled more completely in the time interval between 
letters than in the interval between elements of a 
letter. For this reason “inrush keying” systems 13 
were later developed by means of which a higher 
than normal voltage is applied to the filament at the 
beginning of each element. These methods, de¬ 
scribed more fully in Chapter 5, result in improved 
“crispness” of the transmitted code symbol ele¬ 
ments. 

Operational tests of this system indicated the 
desirability of devising means by which the receiv¬ 
ing ship might break in at any time during the 
transmission of a message. With the beacons just 
described this is impossible, because although the 
signal flux emitted during the intervals between ele¬ 
ments, letters, or words when the filament is cooling 
is small in comparison with the signal flux emitted 
when the filament has reached its equilibrium oper¬ 
ating temperature, nevertheless the signal from the 
local source during these intervals is quite large in 
comparison with a signal from the receiving ship. 
The modulated radiation emitted from the local 
source during the cooling time of the filament there¬ 
fore blocks out reception from a distant source and 
prevents break-in by the receiving operator. For 
this reason a radically different type of beacon was 
developed. It is described in the following section, 
while associated changes in receiver-amplifier de¬ 
sign which were necessitated to secure the feature of 
two-way break-in operation are described in Chap¬ 
ter 5. 

The Type D-2 Beacon 13 

The type D-2 beacon was developed to pennit 
code transmission at a higher speed than was pos¬ 
sible with the type D and to eliminate modulation 
of the local beacon during filament cooling time 
between code elements so that it would be possible 


for a distant operator to break in at any time dur¬ 
ing the transmission of a message. It is seen that the 
second objective might be achieved by direct elec¬ 
trical modulation of the source, since modulation 
would cease as soon as the lamp current was inter¬ 
rupted by releasing the transmitter key. However, 
in order to achieve a high modulation ratio (see 
Section 1.1.4} when it is desired to modulate the 
radiation from a tungsten lamp by direct operation 
from a modulated power supply, even at a low audio 
frequency such as 90 cycles per second, the thermal 
inertia of the filament must be reduced to the lowest 
practicable point through the use of extremely fine 
wire. For example, if it is assumed that the rate of 
cooling of the filament is proportional to the area 
of the incandescent surface, it follows that the ratio 
of the rate of cooling to the heat capacity of a cylin¬ 
drical filament is inversely proportional to the ra¬ 
dius of the filament, and that this ratio will continue 
to increase as the radius of the filament is reduced 
to the smallest practicable value. If an adequately 
high percentage modulation of radiation from an 
incandescent filament is to be achieved by direct 
electrical means at any given frequency, the filament 
must cool down during each cycle to a temperature 
such that the thermal radiation emitted at this 
temperature will be quite small compared with that 
emitted at the peak temperature, and this cooling 
must occur rapidly enough to require only a por¬ 
tion of the cycle. Conversely, such a filament will 
rise quite rapidly to its peak equilibrium tempera¬ 
ture in the remaining portion of the cycle during 
which power is supplied. The incandescence time can 
sometimes be reduced by altering the waveform of 
the power supply to permit momentary overvoltage 
on the filament, thus increasing the portion of the 
cycle available for nigrescence. 

Preliminary experiments indicated that commer¬ 
cially available lamps of the 6S6 and 10S6 types 
could be used successfully as current-modulated 
radiation sources at frequencies up to 90 cycles per 
second. The lamp finally used for this purpose is the 
10 S6 type (230 volts, 10 watts, C-17 filament, S-6 
bulb, intermediate screw base). A lumen output 
rating for this particular lamp is not available, but 
a comparison with values for similar lamps indi¬ 
cates that it should be about 60 to 65 lumens. The 
rated life is 1,500 hours. The mode of operation 
used in the electrically modulated type D-2 beacon 
may affect the service life of the lamp to some ex- 



INCANDESCENT TUNGSTEN LAMPS 


17 


tent, although preliminary tests indicated that any 
such effect is not large. 

A thyratron-controlled power supply was espe¬ 
cially designed and constructed to operate these bea¬ 
cons. Under manual or automatic key control, power 
from the three-phase, 60-cycle ship supply is con¬ 
verted to current pulses for operating the 10S6 
lamps at a frequency of 90 cycles per second. By 
means of controls provided in the power supply, 
the shape and the duration of the current pulses 
can be adjusted to provide a maximum percentage 
modulation of the emitted radiation. The power 
input to the beacons may also be varied as desired. 
It has been found that satisfactory operating char¬ 
acteristics are retained, including reasonably long 
lamp life, when the power input to the lamps is 
up to thirteen per cent greater than their nonnal 
power rating for use on direct current or symmet¬ 
rical alternating current. 



Figure 2. Contour of reflector for the type D-2 
beacon. 


A type D-2 beacon unit contains fifteen lamps 
mounted in a row along a specially designed re¬ 
flector having the cross section shown in Figure 2. 
The reflector, which has an Alzak surface, is made 
of sheet metal bent to form seven plane reflecting 
surfaces so oriented as to give a nearly uniform 
beam intensity distribution within the zone com¬ 
prising 30 degrees above and below the lengthwise 
plane of symmetry of the reflector. Thus a ship on 
which the beacon is installed may roll through any 
angle up to 30 degrees from the vertical without 


appreciably affecting the intensity of the signal 
transmitted to the receiving station. The details of 
construction for one beacon unit are shown in Fig¬ 
ure 7 of Chapter 5. Eight beacon units, mounted 
with the long dimension horizontal and oriented in 
octagon array, provide uniform azimuthal coverage 
over the entire horizon. 

Each beacon is covered with an appropriate infra¬ 
red transmitting filter. The filter having the most 
efficient optical properties for this purpose is either 
a Polaroid polyvinyl alcohol film, code XRN5PX65, 
sandwiched between two clear glass plates, or Ohio 
State University filter material, code DR23u, coated 
on one side of a clear glass plate. Either of these 
filters provides a visual security distance of about 
500 feet and reduces the response of the receiver 
about 48 per cent. The best all-glass filter is Corning 
2568, 8 ± 0.5 millimeters thick. Such a filter pro¬ 
vides a visual security distance between 400 and 
1,200 feet and reduces the signal response of the re¬ 
ceiver about 69 per cent. The general characteris¬ 
tics of these various types of filters are discussed in 
Chapter 2. 

An octagon array of these beacons contains 120 
lamps and has a normal power rating of 1,200 watts. 
The signal intensity from such an array operated 
at 1,200 watts from the type D-2 source power sup¬ 
ply was found by test to be about 12 per cent 
greater than that of a type D beacon containing a 
single 500-watt lamp designed for it, when both 
beacons were covered with approximately 8 mm of 
Corning 2568 infrared transmitting filter glass. 
However, the type D-2 beacon, being an extended 
source, can be covered with a less dense filter than 
the type D beacon while the same visual security 
range is maintained. This possibility, if combined 
with operation of a type D-2 beacon at a 13 per 
cent power overload, results in a ratio of modulated 
radiant signal intensity to electrical power input 
about 70 per cent as large as for the mechanically 
modulated type D beacon. The type D-2 not only 
has no mechanical moving parts but also makes 
possible the break-in feature of operation between 
two stations, since as soon as the key is released at 
the transmitting station there is no modulated local 
signal radiation to interfere with reception of the 
much weaker signal from the distant station. The 
operator of the distant station may break in at any 
time merely by depressing and holding down his 
key. The local operator at once observes that his 


RlfSf+ffeCEIT 











18 


NEAR INFRARED SOURCES 


monitor signal lamp is no longer following his own 
signal transmission and interrupts his transmission 
to await the message which the distant operator 
wishes to interpolate. The features of receiver de¬ 
sign which were necessary to achieve this break-in 
feature in the operation of a complete system are 
described in Chapter 5. 

In field tests of the complete type D-2 system 
made at the Bureau of Ships test station, Fort Miles, 
Cape Henlopen, Delaware, in March 1945, a signal¬ 
ing speed of 30 words per minute was established, 
the speed being limited by the skill of the operator 
rather than by limitations of the equipment. Satis¬ 
factory operation of the break-in feature was also 
demonstrated. Additional laboratory tests were 
made using an automatic keying device to code the 
type D-2 sources and a recorder system to register 
the output of the type D-2 receiver. The recorded 
signals were readable up to a code speed of 40 
words per minute, which was the maximum speed 
obtainable with the keying device. In view of the 
excellent results achieved with the type D-2, Bu- 
Ships negotiated direct contracts for the quantity 
production of the beacons and other components of 
this system. 

Source for Plane-to-Plane Recognition System 14 

The development of a modulated, coded, infrared 
plane-to-plane recognition system was carried 
through the stage of laboratory tests on preliminary 
equipment by the University of Michigan under 
Contract NDCrc-185. The lamp used as the source 
in this equipment is similar to the 10S6 lamp used 
in the type D-2 beacon, differing only in its voltage 
and power ratings, and is operated in a manner 
closely analogous to that utilized for the type D-2 
beacon. 

Each source consists of one 6-watt, 115-volt, type 
6S6 incandescent lamp mounted without a reflector 
in a small aircraft wing nacelle of the type used for 
navigation lights. The nacelle cover is coated with 
a layer of either Polaroid or the Ohio State Univer¬ 
sity infrared transmitting filter having an optical 
density such that the visible range of the lamp is 
less than 100 feet. Complete spherical coverage is 
provided by mounting one source above and one 
below each wing tip. The sources are operated from 
the 800-cycle power supply of Navy aircraft and 
modulated by interrupting the current to the lamp 
90 times per second by means of a motor-driven 


commutator. An automatic coder which controls the 
application of power to the lamps makes it possible 
to transmit any one of some four hundred different 
two-letter code combinations. When operated in this 
manner with a power input of six watts, each source 
is equivalent to a bare 4-candlepower tungsten lamp 
modulated by a rotating disk having equal opaque 
and transparent sectors. A 30 per cent increase in 
signal intensity may be achieved by a 13 per cent 
increase in the power supplied to each lamp. Al¬ 
though the life of the lamp would be somewhat 
reduced by such an overload, it would still be ade¬ 
quately long for this application. 

The complete recognition system of which this 
source constitutes one component is described in 
Chapter 5. 

13 GASEOUS DISCHARGE LAMPS 
131 Flash Lamps 

Early Designs 

All the flash lamps described in this chapter were 
developed primarily by the University of Michigan 
under Contract NDCrc-185. They were first investi¬ 
gated as possible sources of near infrared radiation 
in apparatus for the detection of night-bombing 
planes. 15 Experimental lamps were constructed in a 
variety of shapes and sizes. Some were air-cooled, 
some were water-cooled; some had mercury elec¬ 
trodes, some had tungsten or other solid metal elec¬ 
trodes; some had straight capillary discharge tubes, 
in some the discharge occurred through one or two 
helical spiral coils. However, the best infrared radia¬ 
tion characteristics were invariably obtained when 
the lamps were filled with one or more of the rare 
gases, especially argon, krypton, or xenon. In air¬ 
cooled lamps, regardless of whether the outer en¬ 
velope of the lamp was of glass or quartz, it was 
necessary to provide ballast volume to permit non¬ 
explosive expansion of the gases suddenly heated in 
the high-energy discharge region of the lamp. De¬ 
spite rather stringent limitations on flashing rate 
and energy input per flash, these early lamps showed 
sufficient promise as sources of near infrared radia¬ 
tion to warrant their future specialized development 
for other specific applications later requested by the 
Armed Services. However, the military requirements 
for these applications restricted the later develop¬ 
ments to lamps of the air-cooled type. 


PJTSTDTrTPTT 

IfflJJ X lllxi X El/ 




GASEOUS DISCHARGE LAMPS 


19 


The Type 10 High-Intensity Flash Lamp 16 ' 17 

The type 10 high-intensity flash lamp was de¬ 
veloped in response to an informal request by the 
Army Air Corps for a source suitable for illuminat¬ 
ing the target area in connection with guided-missile 
bombing. The principal requirements outlined in 
the request were: (1) the peak instantaneous flux 
emitted in the near infrared should be as large as 
possible; (2) the lamp should not require forced 
cooling; (3) the firing circuit and associated equip¬ 
ment should be kept as light and compact as pos¬ 
sible; (4) the operating potentials should be kept 
as low as possible to avoid spark-over trouble under 
high-altitude conditions; (5) the dimensions of the 
lamp should be kept small in order that it might 
provide a narrow beam of high intensity when used 
in conjunction with a suitable*reflector. 


provide a means for external triggering control. 
After being outgassed, the lamp is filled with a mix¬ 
ture consisting of 90 per cent krypton and 10 per 
cent xenon to a total pressure of 20 to 70 centimeters 
of mercury. The intensity of the flash is almost in¬ 
dependent of pressure within this range, and since 
the lamp is triggered by means of a third electrode 
the voltage to which the firing condensers must be 
charged is also unaffected by the gas pressure. 

Method of Firing. The normal method of firing 
this lamp utilizes a Tesla coil to pass a high-voltage 
spark from the third electrode to one or both of the 
principal electrodes. A 25-pf condenser connected 
across the lamp terminals then discharges through 
the conducting gas, producing the high-intensity 
flash. The condenser is charged to a potential of 
1,000 to 2,000 volts from a suitable power supply 
with a protecting resistor in series. The rate of flash- 



Design of the Lamp. The design of the type 10 
flash lamp is shown in Figure 3. The discharge is 
confined to a length of 4 centimeters within a quartz 
capillary tube of 4 millimeters inside diameter and 
2 millimeters wall thickness. The main electrodes 
are of iron, turned to fit snugly into the ends of the 
quartz tube and supported on 100-mil tungsten 
leads. The life of the lamp is materially lengthened 
if the electrodes are tipped with alloy electrode pel¬ 
lets of low work function obtained from the firm of 
Edgerton, Germeshausen and Grier, Cambridge, 
Massachusetts. The outer envelope, made of Nonex 
glass, is approximately 8 centimeters long and 2 
centimeters in outside diameter. The third electrode 
consists of a 30-mil tungsten wire sealed into the 
envelope with one end extending into a small hole 
blown in the center of the quartz discharge tube to 


ing may be manually controlled, or the lamp may be 
fired automatically at a rate up to three flashes per 
second by means of a motor-driven cam and switch 
arrangement used to energize the primary of the 
Tesla coil. 

Radiation Characteristics. Both the duration and 
the peak intensity of the flash are affected by the 
constants of the electric circuit which is used to fire 
the lamp. The values given in Table 1 were obtained 
with the lamp connected by short leads to a 25-pf 
condenser. 

The intensity versus time characteristics of the 
flash have been investigated by a photographic 
method utilizing a rotating mirror arrangement and 
by a special oscillograph utilizing a cesium-surface 
vacuum phototube as the detector. The peak inten¬ 
sity is attained 6 to 8 psec after the beginning of 





































20 


NEAR INFRARED SOURCES 


the flash, while the total elapsed time from the be¬ 
ginning of the flash until the intensity has decayed 
to about 15 per cent of the peak value is 30 to 
35 psec. 

The peak hololuminous intensity versus energy 
input per flash in a direction perpendicular to the 
axis of the lamp, measured with reference to the 
cesium-surface vacuum phototube, is shown in 
Table 1. Up to an input energy of about 20 joules 


Table 1. Energy input per flash and peak intensity 
of the type 10 lamp relative to a cesium-surface 
vacuum phototube. 


Firing 

potential 

(volts') 

Energy input from 
25-(xf condenser 
(joules) 

Peak intensity 
(equivalent 
holocandles) 

1185 

17.6 

1.0 X 10 6 

1300 

21.1 

1.2 X 10 6 

1815 

41.2 

2.2 X 10 6 

2060 

53.1 

2.7 X 10 6 


per flash the lamp may be fired continuously at the 
rate of three flashes per second without excessive 
overheating. Above 20 joules per flash a lower rate 
of firing is recommended. If the power input be¬ 
comes too high the quartz discharge tube becomes 
so hot that the triggering circuit no longer controls 
the rate of firing, and the life of the lamp is short¬ 
ened. For applications in which larger dimensions 
could be tolerated, the permissible power input 
might be correspondingly increased without the 
necessity for adding forced cooling methods. 

The beam holocandlepower at the peak of the 
flash and the angular distribution of intensity in the 
beam have been measured with the type 10 lamp 
mounted lengthwise on the axis of a glass second- 
surface precision parabolic reflector of 19% 6 -inch 
aperture and 7%-inch focal length. These measure¬ 
ments were made for five different positions of the 
lamp along the axis of the reflector, and the results 
are shown in Figure 4. For curve 1, three sets of 
experimental points are shown. These represent the 
angular distribution of intensity obtained with the 
center of the lamp at the focal point, and at a dis¬ 
tance of 1 centimeter from the focal point along the 
axis both toward and away from the reflector. The 
distribution is so nearly identical for these three 
positions of the lamp that only one curve has been 
drawn for the three separate sets of points. For 
curves 2 and 3, the two extremities of the discharge 


were located in turn at the focal point of the reflec¬ 
tor; for curve 2 the lamp extended toward the reflec¬ 
tor from the focal point, while for curve 3 the lamp 
extended away from the reflector beyond the focal 
point. The peak axial beam holocandlepower of 
about 1.6 billion shown in the figure is for an energy 
input of 41.2 joules per flash, with the lamp operated 
under the same conditions and using the same de¬ 
tector as for the corresponding entry shown in 
Table 1 for the bare lamp without reflector. 



DEGREES FROM AXIS OF SYMMETRY 

Figure 4. Intensity distribution across the beam pro¬ 
jected from a type 10 lamp mounted in a 19%6-inch 
reflector of 7%-inch focal length, with reference to a 
cesium-surface detector. 

Life tests have been made with the lamp flashed 
three times per second at an input energy of 21 
joules per flash. For a bare lamp with iron elec¬ 
trodes, the peak flash intensity is reduced to 75 
per cent of the initial value after about 3,000 flashes, 
or 15 to 20 minutes of continuous operation. For a 
bare lamp with the special alloy electrode pellets 
referred to above, the corresponding figures are 
22,000 flashes, or about two hours of continuous 
operation. Under these same firing conditions, the 








































GASEOUS DISCHARGE LAMPS 


21 


infrared component of the radiation decays much 
less rapidly with time than the peak intensity of the 
total lamp radiation to which the phototube re¬ 
sponds. For example, when the lamp is covered with 
a Polaroid XR3X44 filter, the lamp may be flashed 
about 8,000 times, corresponding to about 45 min¬ 
utes of continuous operation, before the peak inten¬ 
sity of the transmitted component of the radiation 
is reduced to 75 per cent of its initial value. 

Table 2 shows the ehT values (see Appendix) of 
certain infrared transmitting filters for the radiation 
from a new type 10 flash lamp, with respect to a 
cesium-surface vacuum phototube. The plastic mem¬ 
brane filters were supported between two clear glass 


Table 2. Effective holotransmission of certain filters 
with reference to a cesium-surface detector. 


Filter 

ehT for 
radiation from 
type 10 lamp 

ehT° for radia¬ 
tion from 2848 K 
tungsten source 

Wratten 87 

0.11 

0.38 

Polaroid XR3X44 

0.043 

0.19 

Polaroid XR7X30 

0.025 

0.13 

Corning 2600 5.0 mm 

0.15 

0.36 

Coming 2550 2.0 mm 

0.073 

0.27 

Corning 2540 2.6 mm 

0.024 

0.13 


lantern slide plates for the absorption and reflection 
of which no correction has been made. For compari¬ 
son, the standard values (ehT°) for the same filter 
samples with respect to radiation of standard dis¬ 
tribution from a 2848 K tungsten source are also 
given. Somewhat higher ehT values than those listed 


A small quantity of lamps based on the Michigan 
type 10 design was constructed by the Westing- 
house Electric Corporation for the Army Air Forces. 
It is believed that facilities for large-scale commer¬ 
cial production could readily be set up if desired, 
and that the fundamental design features of the 
lamp could be easily adapted to other shapes and 
sizes which might be required for specific applica¬ 
tions of a source of this type. 

The Type 200 Microflash Lamp 18 > 19 

The type 200 microflash lamp and an associated 
firing unit for use in ballistic photography were de¬ 
veloped at the request of Aberdeen Proving Ground 
as Project Control OD-147. In order to “stop” com¬ 
pletely a high-speed projectile it is essential that the 
duration of the flash be no more than a few micro¬ 
seconds; it is also essential that the lamp be trig¬ 
gered through an arrangement which permits 
synchronization of the flash with the shell trajec¬ 
tory and that an area sufficiently large to “catch” 
in flight projectiles up to 12 or 16 inches in diameter 
be illuminated at a sufficiently high intensity to 
register on the film. These requirements have been 
successfully met with the equipment described be¬ 
low. 

Although the type 200 lamp has been used pri¬ 
marily as a source of visible radiation for ballistic 
photography, it is in part an outgrowth of the type 
10 lamp described above and, in turn, contributed 
to the development of the type 300 microflash lamp 
described in a subsequent section. 



are obtained when the lamp is operated at a lower 
energy input per flash. Moreover, because of the 
slower rate of decrease for the infrared component 
of the radiation mentioned in the preceding para¬ 
graph, the ehT values tend to increase during con¬ 
tinued use of the lamp. 


Design of the Lamp. The general design of the 
lamp, together with the recommended dimensions, 
is shown in Figure 5. The discharge is confined to 
the annular space between the Pyrex rod and the 
concentric cylindrical Pyrex glass envelope which 
surrounds it. The short duration of the flash appar- 

































22 


NEAR INFRARED SOURCES 


ently results from recombination of ions at the solid 
surfaces which are everywhere closely adjacent to 
the discharge. By providing ballast volume to per¬ 
mit expansion of the gas heated by the discharge, 
the bulbs at the ends of the lamp tend to quench 
the oscillations which may occur in the discharge 
and in the resulting radiation and also help to 
lengthen the life of the lamp. 

If the energy input to the lamp is to be high (of 
the order of 25 joules per flash), it is important that 
the wall thickness of the Pyrex tube from which the 
outer envelope is made be at least 2 millimeters. 
Lamps constructed of quartz would have much 
greater mechanical strength, but the Pyrex construc¬ 
tion is much simpler and ordinarily provides ade¬ 
quately long life. The life of a lamp depends 
critically upon the conditions of use and may be as 
short as twenty flashes at 25 joules per flash. Care¬ 
ful annealing of the tungsten seals is also an impor¬ 
tant factor in lamp life. 

The lamp is filled with a gas mixture consisting 
initially of 90 per cent krypton and 10 per cent 
xenon, enough hydrogen being added to enable the 
lamp to withstand a given firing condenser voltage 
until it is triggered. To withstand 11,000 volts, for 
example, enough hydrogen must be included to con¬ 
tribute about 25 per cent of the total pressure of 
45 to 55 centimeters of mercury to which the lamp 
is filled. The hydrogen also helps to suppress oscil¬ 
lations in the discharge. An external loop or band of 
wire around the middle of the lamp serves as a satis¬ 
factory triggering electrode for use with a spark coil 
or a Tesla coil. 

Method of Firing. Since a flash of short duration 
can be obtained only if the time constant of the 
associated electric circuit is kept small, it is neces¬ 
sary to fire the lamp from a condenser of relatively 
low capacitance and to achieve a high energy input 
per flash through the use of a correspondingly high 
voltage. In the microflash unit, therefore, the lamp 
is fired from a 0.5-pf condenser, consisting of two 
1 .0-pf condensers in series, charged to a maximum 
potential difference of about 11,000 volts. The con¬ 
denser is charged by a suitable voltage-doubler 
power supply. To trigger the lamp at the proper 
time for photographing a projectile the output from 
a suitably located microphone, photocell, or electro¬ 
static loop is amplified and applied to the grid of a 
thyratron tube, which then discharges a condenser 
through the primary of a spark coil. The resulting 


electric pulse in the secondary of the coil, which is 
connected to the triggering loop around the lamp, 
initiates the flash. 

The life of the lamp will be extended by operation 
at the lowest voltage which will provide a flash of 
the required intensity. The lamp is designed only 
for intermittent use, as in ballistic photography, and 
will not withstand successfully a high rate of firing 
at the large energy input per flash needed to produce 
the required radiant intensity. 

The complete microflash unit, consisting of a type 
200 lamp, a 12-inch parabolic reflector with Alzak 
diffusing surface, the trigger circuit, the lamp firing 
circuit, and a power supply unit designed to operate 
from a standard 115-volt, 60-cycle power line, is 
compactly mounted in a portable case approxi¬ 
mately 10x13x17 inches. 

Radiation Characteristics. The duration of the 
flash has been measured by two independent meth¬ 
ods. When the rapidly moving image projected from 
a rotating mirror is recorded on orthochromatic film, 
a photographic duration time of 1 to 2 psec is ob¬ 
tained. When the duration is measured with a ce¬ 
sium-surface vacuum phototube and a special oscil¬ 
lograph having a high-speed calibrated sweep, a 
value of 4 to 5 psec is observed from the beginning 
of the flash until the intensity has fallen to about 15 
per cent of the peak value. The difference in the 
results of the two methods is attributed to the 
tendency of the photographic film to respond only 
to the highest intensity portion of the flash, thereby 
failing to record an undetermined portion of the 
total duration. 

When the lamp is fired from a 0.5-pf condenser 
charged to 10,000 volts (25 joules input per flash) the 
peak intensity of the flash is about 5 X 10 6 equiv¬ 
alent holocandles with respect to a cesium-surface 
detector; the peak hololuminous intensity value 
would probably be considerably higher for a de¬ 
tector having the spectral response characteristic of 
orthochromatic film. The ehT values for near infra¬ 
red filters with reference to the type 200 radiation 
and a cesium-surface detector are only about one- 
half as large as the values shown in Table 2 for 
radiation from the type 10 lamp. It is possible that 
the discrepancy in flash duration measurements by 
the two methods outlined above may be partly due 
to differences in the duration characteristics of the 
high-intensity visible component of the radiation to 
which the orthochromatic film responds, and the 





GASEOUS DISCHARGE LAMPS 


23 


near infrared radiation of much lower intensity to 
which the cesium-surface phototube principally 
responds. 

In use the lamp is mounted coaxially along the 
axis of a 12-inch diffusing surface parabolic reflector 
of 2.5-inch focal length with the center of the lamp 
approximately at the focal point of the reflector. 
The illumination provided by this arrangement is 
sufficient for ballistic photography over the entire 
field (30 to 40 degrees) of a camera placed near the 
microflash unit. 

Performance. After their superior performance 
characteristics had been established in preliminary 
trials, four complete microflash units and an addi¬ 
tional supply of type 200 microflash lamps were 
delivered to the Ballistic Research Laboratory, 
Aberdeen Proving Ground, in accordance with the 



Figure 6. Six-inch projectile photographed with the 
type 200 microflash unit. 


request made in Project Control OD-147. The per¬ 
formance of both lamps and microflash units has 
apparently been satisfactory in every respect. A 
sample photograph of a 6-inch shell in flight at ap¬ 
proximately 2,800 feet per second is shown in Fig¬ 
ure 6. Chalk was rubbed on the shell to improve its 
contrast with the background. The photograph was 
taken by reflected light at a distance of about 150 
feet from the muzzle of the gun. It is seen that both 
the translational and the rotational motions of the 
shell were completely stopped. The microphone used 
to trigger the flash from the bow wave of the shell 
is seen at the lower right. 

As a result of the superior performance of the 
microflash unit, the Army Ordnance Department 
has completed preliminary negotiations for the com¬ 


mercial production of a supply of type 200 lamps, 
and possibly also for the production of a number of 
additional complete microflash firing units. 

Related Developments. Two related developments 
have been made by the University of Michigan for 
use in ballistic photography. One is a device 20 to 
trip a camera shutter automatically at just the 
proper moment to catch a projectile in flight when 
it is illuminated \>y the microflash unit. This device 
permits ballistic photographs to be made during the 
day or on a lighted range at night. This develop¬ 
ment was made on the basis of an informal request 
by the Army Ordnance Department during the 
course of the work on the microflash unit under the 
project previously mentioned. The second develop¬ 
ment 21 was made as Project Control OD-173. It 
consists of an adaptation of the type 10 lamp to 
illuminate a clock face at just the proper moment 
so that its image will be registered on each frame 
of a DeBrie high-speed movie camera used by the 
Aberdeen Proving Ground. 

The Type 300 Microflash Lamp 22 

The type 300 microflash lamp was produced at the 
Lamp Development Laboratory of the General 
Electric Company with the consultation and assist¬ 
ance of the University of Michigan flash lamp 
group, to serve as a pulsed radiation source in 
equipment for detecting the range and direction of 
certain types of targets by means of near infrared 
radiation (see Chapter 6). It was initially desired 
that the source be capable of continuous operation 
at the rate of 60 to 120 flashes per second for periods 
of several minutes at a time, that its integrated 
useful life be at least several hours, that the ampli¬ 
tude of successive radiation pulses be essentially 
constant, and that each pulse have a duration of 
only about 1 psec and the highest possible peak 
intensity in the near infrared. The type 300 lamp 
successfully meets all these requirements and has 
been used in models of the equipments for which its 
development was undertaken. 

Two preliminary lamp designs, 22 " designated 
types 240 and 250, were developed by the Univer¬ 
sity of Michigan. In each of these designs, as in the 
type 10 and in the type 200 lamp designs, the arc 
discharge is adjacent to a solid quartz surface. Al¬ 
though the radiation characteristics of these lamps 
are quite similar to those described below for the 
type 300 lamps, both designs were discarded because 




24 


NEAR INFRARED SOURCES 


the useful life of these lamps was too short for 
satisfactory service in the intended application. 

The designation “type 300 microflash lamp” has 
been used only by NDRC. The General Electric 
Company has so far designated the lamp simply a 
“short-gap double-ended lamp.” In this chapter, 
however, no distinction will be made between the 
lamps constructed by the General Electric Company 
and those constructed by the University of Michi¬ 
gan Contract NDCrc-185. Most of the varia¬ 
tions in lamp design for the purpose of investigating 
the effect of various factors on the properties of the 
lamps, as well as the experimental investigation of 
these properties, were carried out by the University 
of Michigan group. 



PYREX BULB 

Figure 7. The type 300 microflash lamp. 


Design of the Lamp. The design of the type 300 
lamp and the standard dimensions used by the Gen¬ 
eral Electric Company are shown in Figure 7. The 
design is simple and lends itself readily to large- 
scale production techniques. An important feature 
in securing long operating life is the use of the low 


work function alloy pellets, mentioned above in 
connection with the type 10 lamp, with which each 
electrode is tipped. After the lamp has been evacu¬ 
ated and outgassed it is filled with one or more of 
the rare gases, plus 1 to 3 centimeters of hydrogen 
to increase the self-breakdown potential at which 
the lamp will fire. The total pressure may vary from 
50 to 70 centimeters of mercury. Argon is the filling 
gas principally used by the General Electric Com¬ 
pany. 

In the lamps constructed at the University of 
Michigan the main steel electrodes are supported 
on 100-mil tungsten leads sealed into a Nonex or 
Pyrex envelope, with brass electrode caps brazed to 
the outer ends of the tungsten seals. In order to in¬ 
crease the intensity of the flash by increasing the 
firing voltage of the lamp are gaps up to 10 mm 
long have been used in some lamps as compared 
with the 3-mm gap of the standard General Electric 
design. With the longer gaps, it was found to be 
advantageous to decrease the diameter of the steel 
electrodes from % inch to % 6 inch and to round off 
the sharp corners of the bevels at the tip of each 
electrode. Argon, krypton, and xenon were used as 
filling gases, both singly and in mixtures. A mixture 
of argon and krypton in approximately equal vol¬ 
umes was found to produce the most desirable over¬ 
all characteristics. However, the advantage of this 
mixture over pure argon is probably too small to 
make its use economically justifiable. 

Method of Firing. In use, the lamp is connected 
directly across the terminals of a 0.1-pf condenser 
(a resistance of 0.5 ohm or less may be in series). 
The condenser is charged through a suitable pro¬ 
tective resistor from a half-wave or full-wave un¬ 
filtered power supply, and the lamp is fired when the 
voltage across the condenser builds up to the break¬ 
down potential of the lamp. It has been found 
advantageous to mount the lamp directly on one 
terminal of the condenser inside a coaxial cylindrical 
shield of brass or copper connected to the outer end 
of the lamp so as to serve as the second lead from 
the condenser. This arrangement provides good elec¬ 
tric and magnetic shielding from the large peak 
discharge current (of the order of 2,000 to 4,000 
amperes) and also provides a good means for dis¬ 
sipating the heat generated in the lamp. If the power 
supply is operated from a 60-cycle line, the lamp 
can be fired either 60 or 120 times per second with¬ 
out an auxiliary triggering system. The voltage at 


nE S TOicfEir 




















































GASEOUS DISCHARGE LAMPS 


25 


which the lamp fires is determined by the details 
of its construction and the characteristics of the 
filling gas. Most lamps have been made to fire at 
voltages between 3,000 and 4,000 volts. The firing 
of the lamp may be controlled by means of a Variac 
in the primary circuit of the power transformer. 
By adjusting the voltage applied and the constants 
of the lamp-firing circuit so that the condenser is 
not charged to the full breakdown voltage of the 
lamp on each half cycle, it is possible to fire the 
lamp at integral submultiples of 60 times per second. 
Type 300 lamps have been operated continuously at 
120 flashes per second for many hours. The flash 
repetition rate could probably not be increased much 
beyond this value without necessitating means for 
forced cooling of the lamp, which has hitherto been 
considered undesirable. As a consequence, this rate 
of flashing sets an upper limit to the feasible scan¬ 
ning rate for systems in which the lamp is used. 

Radiation Characteristics. The arc discharge 
which occurs in the type 300 lamp consists of a thin 
central core of very high brightness surrounded by 
a sheath of lower brightness about 3 mm in diameter. 
Tests conducted by Western Electric Company 
(Bell Telephone Laboratories) Contract OEMsr- 
1267 indicate that the brightness of the cen¬ 
tral core is 10 to 20 times greater than that of 
the surrounding diffuse sheath and that the ehT 
values of infrared filters for radiation from the core 
are three or more times the values found for the 
radiation from the entire arc. The data given in 
Tables 3 and 4 refer to average values for the radia¬ 
tion from the entire arc in a number of representa¬ 
tive lamps. 

The data on lamp characteristics were obtained 
at the University of Michigan with a cesium-surface 
vacuum phototube coupled through a wide-band 
amplifier to the vertical deflection plates of a cath¬ 
ode-ray tube. This receiver was calibrated by using 
the radiation from a standardized tungsten lamp 
chopped by means of a high-speed slotted disk. The 
high-speed horizontal sweep of the cathode-ray tube 
was triggered by means of the lamp discharge cur¬ 
rent, so that a stationary pattern of the intensity 
versus time characteristics of the lamp radiation was 
presented on the screen of the cathode-ray tube. 
The lamps were fired 60 times per second from a 
0.1-pf condenser. For a lamp which breaks down 
at 4,000 volts, this corresponds to an energy input 
of 0.8 joule per flash or a power dissipation of 48 


watts in the lamp. The peak intensity of the first 
few flashes may be 20 per cent higher than the 
equilibrium values measured after the first few 
seconds of operation. Some typical average charac¬ 
teristics are summarized in Table 3. The duration 

Table 3. Average radiation characteristics of repre¬ 
sentative type 300 microflash lamps. Capacitance of 
firing condenser, 0.1 pi. Firing potential, 3,000 to 4,000 
volts for 3- to 4-mm arc length, 4,000 to 8,000 volts for 
7- to 10-mm arc length (depending on arc length and 
on pressure and composition of the filling gas). 




Peak intensity relative to a 
cesium-surface detector 
(equivalent holocandles) 

Arc 

length 

(mm) 

Duration from 
beginning of flash 
down to 15 per 
cent of peak 
intensity (psec) 

Bare lamp 

Lamp mounted in 
lSP^-in. precision 
parabolic reflector 

3-4 

7-10 

1.6- 2.4 

1.7- 2.7 

(0.7-2) X 10 5 
(1.8-5) X 10 5 

(2^.5) X 10 8 
(1-2.5) X 10 9 


of the flash between the points of half peak intensity 
is between 0.5 and 0.75 psec for most lamps, the 
much longer duration values down to 15 per cent of 
the peak intensity, shown in the second column of 
the table, being due principally to the gradual decay 
of intensity at the “tail” of the curve. The beam 
candlepower values given in the last column of the 
table w r ere obtained with the arc transverse to the 
axis and centered at the focal point of a second- 
surface precision glass parabolic reflector of 7%-inch 
focal length and 19%6-4 nc h aperture. With a lamp 
having a 1-centimeter arc the angular dimensions of 
the beam projected from this reflector are approxi¬ 
mately 1x3 degrees. Since the bright central core 
may be displaced by an amount greater than its 
own width through the effects of thermal convection 
currents, electrode surface irregularities, etc., suc¬ 
cessive flashes do not follow the same path with 
enough exactness to permit good measurements to 
be obtained for the intensity distribution across the 
beam. 

The spectral intensity distribution of the radia¬ 
tion from these lamps has not been measured. 
However, as in the case of other flash lamps, it un¬ 
doubtedly consists of the characteristic line spec¬ 
trum of the filling gas superposed upon a continuous 
spectrum. The average ehT values of certain filters 
for radiation from the type 300 lamps with reference 








26 


NEAR INFRARED SOURCES 


to a vacuum cesium-surface detector are given in 
Table 4. These values are relatively independent of 
which of the gases indicated above is selected for 
filling the lamp. The highest ehT values for radia¬ 
tion from the entire arc were obtained with argon- 
filled lamps, while the highest values for radiation 
from only the central bright core of the arc were 
obtained with xenon-filled lamps. No correction has 
been made for the reflection and absorption of the 
two lantern-slide mounting plates between which 
the plastic membrane filters listed in Table 4 were 
supported. 


Table 4. Effective holotransmission of certain filters 
with reference to a cesium-surface detector. 


Filter 

Average ehT for 
radiation from 
type 300 flash 
lamp 

ehT° for radia¬ 
tion from 2848 Iv 
tungsten source 

Wratten 87 

0.074 

0.38 

Polaroid XR3X44 

0.029 

0.19 

Polaroid XR7X30 

0.017 

0.13 

Corning 2600 5.0 mm 

0.085 

0.36 

Corning 2550 2.0 mm 

0.047 

0.27 

Corning 2540 2.6 mm 

0.017 

0.13 


Lamp Life and Performance. No type 300 lamp 
is known to have been operated either to destruction 
or to the end of its useful life as a radiation source. 
In a life test conducted by the Western Electric 
Company Contract OEMsr-1267, a General Elec¬ 
tric lamp was fired 120 times per second from 
a 0.1-pf condenser with a 0.2-ohm resistor in series 
for a total of 450 hours in continuous periods of 
about eight hours each. This corresponds to nearly 
200,000,000 flashes. After 210 hours the peak inten¬ 
sity of the flash had decreased to about 70 per cent 
of the initial value, due partly to a decrease of the 
lamp firing potential and partly to clouding of the 
glass envelope by material sputtered from the elec¬ 
trodes. 

In view of the long life of these lamps it might 
be supposed that pulses of higher peak intensity 
could be obtained by operating the lamp from a 
larger condenser or at higher firing voltages. How¬ 
ever, the duration of the flash increases much more 
rapidly than its peak intensity when the capaci¬ 
tance of the firing condenser is increased, and the 
radiation characteristics of the lamp become less 
desirable if the construction and processing are al¬ 
tered so as to increase the self-breakdown firing 


voltage appreciably above the range of values shown 
in Table 3. Some increase in peak intensity might 
be obtained if a control tube which would withstand 
higher voltages (such as an ignitron) were added to 
the firing circuit. With the addition of such equip¬ 
ment it might also be possible to fire the lamps at 
considerably higher rates than 120 times per second. 
However, the problems associated with supplying 
adequate electric power and adequate means for 
dissipating the heat developed in the lamp during 
continuous operation at high intensity will undoubt¬ 
edly continue to limit the rate of flashing which is 
feasible. i 

The operation of these lamps in the equipment 
(see Chapter 6) for which they were developed has 
been very satisfactory. It is not known whether the 
General Electric Company plans to produce such 
lamps on a commercial basis but they will presum¬ 
ably continue to be available from this company on 
special order if not otherwise. 

Concentrated-Arc Lamps 

The concentrated-arc lamp 23 is a new type of 
radiation source developed by the Western Union 
Telegraph Company. In May 1943, when its devel¬ 
opment was still in the laboratory stage, Contract 
OEMsr-984 was negotiated with the Western Union 
Company for the further development and study of 
this type of lamp as an electrically modulated source 
of near infrared radiation for communication pur¬ 
poses, primarily in connection with Project Control 
NS-159. 

During the contract period from May 1943 to 
August 1945 the basic theory of operation of the 
lamp was investigated, the design and methods of 
fabrication were improved, the number of useful 
sizes, types of construction, and operating circuits 
was increased, the desired radiation characteristics 
were enhanced, and the average operating life of the 
lamps was lengthened. Nearly 6,000 lamps of vari¬ 
ous types were supplied to the Armed Services and 
to other government agencies and contractors for 
wartime use. The principal application of the lamps 
by NDRC Section 16.4 was their use in the early 
models of an infrared voice and code communication 
system 24 (Navy type E) by Northwestern Uni¬ 
versity Contract OEMsr-990. The active co¬ 
operation of those working under this contract in 
measuring the radiation characteristics and investi- 


RBST ftiG T E D 







GASEOUS DISCHARGE LAMPS 


27 


gating the basic mode of operation of the lamps was 
of material assistance in motivating and accelerating 
the successful improvements in design and construc¬ 
tion of the lamps under the Western Union contract. 
Concentrated-arc lamps were also used by the Army 
in a narrow-beam, battery-operated aural signal 
unit for code and voice communication; by the Navy 
in a hand-held equipment of medium beam width 
for voice communication between airplanes, or be¬ 
tween airplanes and ground stations; by various 
Armed Service agencies and contractors in bore¬ 
sighting equipment; and by the Office of Strategic 
Services in equipment for projecting aerial photo¬ 
graphs from the same angle at which the exposures 
were made. 

The Western Union Telegraph Company has also 
developed manufacturing facilities for lamps of sev¬ 
eral standard types which are now commercially 
available. 25 These standard types are rated at 2, 10, 
25, and 100 watts. Except where otherwise indicated, 
the material which follows refers exclusively to 
lamps of these standard types. 

Lamp Design and Construction 

The name of the concentrated-arc lamp is derived 
from the small cathode spot of high brilliance from 
which, in the standard types of this lamp, the major 
portion of the radiation is emitted. These lamps 
consist basically of two permanent electrodes sealed 
into a glass bulb filled with argon gas at a pressure 


spectral lines which originate from a highly excited 
layer of zirconium vapor and argon gas very near 
the cathode surface. The arc current tends to return 
the zirconium ions to the cathode, thus renewing its 
surface and resulting in a lamp life which may ex¬ 
ceed 1,000 hours when the lamp is not modulated. 
A much shorter life results when the lamp is oper¬ 
ated at a high per cent current modulation. The 
anode is designed for efficient cooling in order to 
minimize the vaporization of material from it which 
might contaminate the cathode. 

By proper cathode design the diameter of the 
radiation source may be made so small as to approxi¬ 
mate a point source. The concentrated-arc lamps 
may possess modulation ratios (Section 1.1.4) and 
brightness values comparable with those of the 
carbon-arc lamp while having the added advan¬ 
tages of nonvaporizing electrodes and a fixed posi¬ 
tion for the cathode spot. 

Figure 8 shows a variety of experimental designs 
which have been successfully constructed, including 
several lamps in sizes larger than standard. Certain 
characteristics of the four standard lamp types are 
summarized in Table 5. In the standard type, the 
100 -watt lamp is available only in a side projection 
model similar to item 7 of Figure 8. The other sizes 
are available either in a side-projection or an end- 
projection model. In all of the standard types the 
anode consists of a metal plate with a hole through 
which the radiation from the cathode emerges. 


Table 5. Operating data on standard types of concentrated-arc lamps. 


Nominal Light Brightness Candle- Max temp 

lamp source candles/mm 2 power (degrees F) 


rating 

(watts) 

Volts 

Amp 

diameter 

(mm) 

max 

avg 

Candle- 

power 

per 

watt 

Life in 
hours 

Bulb 

type 

Base 

type 

bulb 

base 

2 

37 

0.055 

0.085 

96 

56 

0.32 

0.155 

175 

T5 

Min, 3 pin 

140 

100 

10 

21 

0.5 

0.4 

55 

22 

2.7 

0.26 

900 

T9 

Small, 8 pin 

225 

130 

25 

20 

1.25 

0.73 

40 

21 

8.5 

0.35 


T9 

Small, 4 pin 

355 

145 

100 

15.4 

6.25 

1.58 

52 

39 

77 

0.80 

700 

ST19 

Medium, 4 pin 

470 

160 


of one atmosphere. The unique radiation character¬ 
istics are largely due to a specially prepared zir¬ 
conium oxide cathode. The major portion of the 
emitted radiation is a continuum having a spectral 
distribution similar to that of a black body near 
2800 K. Superimposed on the continuum are intense 


Cathode Construction. The cathode consists of a 
tantalum tube packed with finely ground fused zir¬ 
conium oxide and drawn to a cored wire of the 
desired size. A short length of this wire is mounted 
in the lamp so that one end, with its exposed oxide, 
is % 2 to % inch from the anode. When the arc is 








28 


NEAR INFRARED SOURCES 




Figure 8. Experimental types of concentrated-arc lamps. 


RE S TRICTED ^ 

















GASEOUS DISCHARGE LAMPS 


29 


first struck, the oxide fuses to a hemispherical bead 
in the end of the wire. Tantalum is used as the 
sheath because of its high melting point and the 
ease with which it can be worked and drawn. For 
each lamp size this sleeve is proportioned to operate 
at a temperature no higher than 2000 K at the 
hottest point. A still higher operating temperature 
might result in improved efficiency for producing 
steady radiation but would probably be detrimental 
to the modulation characteristics of the lamp and to 
the stability of the cathode spot. The most stable 
and uniform operation is obtained when the cathode 
spot very nearly fills the activated zirconium oxide 
surface. 

Anode Construction. The anode is made of molyb¬ 
denum punched or cut from sheet stock and formed 
to the desired final shape in a small press. Except 
for the difficulty of shaping it, tungsten is an 
equally good anode material. The shape of the 
anode is not important; its dimensions are chosen 
only to provide adequate heat dissipation. The arc 
strikes to the edge of a hole, in the center of the 
anode, which also serves as a window for the cath¬ 
ode. It is found that experimental lamps rated at 
more than 300 watts have better characteristics if 
water-cooled anodes are provided. 

Other Features of Construction. The radiation 
spectrum contains lines characteristic of the gas 
with which the lamp is filled. Argon and krypton 
are the most satisfactory gases, and the former is 
normally used. 

The electrodes may be mounted in any position 
within the bulb so long as their relative orientation 
and spacing are properly preserved. The standard 
point cathode and perforated-plane anode structure 
lowers the breakdown potential required for start¬ 
ing the lamp and also results in lengthened lamp 
life, since the arc may operate from any point on 
the edge of the hole in the anode. 

The surface of the glass bulb must be large 
enough to avoid overheating at any point and may 
be smaller for Pyrex or Nonex than for soft glass 
bulbs. Standard commercial tube- and lamp-type 
bulbs may be used for side-projection models, but 
special measures are ordinarily required to provide 
sufficiently good optical quality for the windows in 
end-projection types. The lamps may be mounted in 
various types of bases, the principal requirements 
being that they withstand the high voltages used in 
starting the lamps and that they be polarized. 


Molded Bakelite bases of the radio-tube type are 
preferred, such as those listed in Table 5. Prefocus 
bases can be provided if they are needed, for exam¬ 
ple, to preserve the alignment of an optical system 
when the lamp is exchanged or replaced. 

Static Electrical Characteristics and Operating 
Circuits 

The volt-ampere characteristic curve for the con¬ 
centrated-arc lamp has a negative slope, just as for 
other well-known arcs. However, if the arc current 
is increased beyond the point at which the cathode 
spot just fills the activated zirconium oxide surface, 
the slope of the volt-ampere curve tends to increase 
to zero and may even become positive. Once the arc 
is established, stable d-c operation requires only that 
sufficient positive ballast impedance be included in 
the circuit to neutralize any negative resistance ex¬ 
hibited by the lamp over the desired operating range 
of current. 



Figure 9. Average static volt-ampere characteristics 
of aged concentrated-arc lamps. Arrows indicate nor¬ 
mal operating current. 


Since the characteristics of a lamp change some¬ 
what during its operating life, the data chosen for 
presentation below are average values for lamps 
aged sufficiently to have well-stabilized characteris¬ 
tics. The average static volt-ampere characteristics 
of the four standard lamp sizes are shown in Fig¬ 
ure 9. Other average static characteristics for lamps 
of the 10-watt size, as an example, are shown in 
Figure 10. 

The concentrated-arc lamps operate on relatively 
low-voltage direct current but require high-voltage 
starting circuits. By means of rectifier power sup¬ 
plies they can be started and run from an a-c source 


lfES33EE£9Sf) 







































































30 


NEAR INFRARED SOURCES 


if desired. Such power units must include a high- 
voltage starting supply, a low-voltage operating 
supply, and adequate provisions for switching the 
lamp from one supply to the other at the proper 
time. 






i 1 

ORMALCL 

JRRENT t 

-T-T 

JGHT SOU 

J . 









DIAMETEI 


ANGLE PC 
PER WAl 

-f— w 

-32 - 








)WER - 

rT 

' K 









2 

£ 

>24 - 









£ 

V 

1 

/ 

/ 








tie jjj — 









8 

8 02- 










i- 

* 



















0> 

1 

o at 

SL 

1 

0.3 

-1-2.1 

AMPERES 

<V»- 1 

0.7 

21 

21 

10 



Figure 10. Average static radiation characteristics of 
aged 10-watt concentrated-arc lamps. 


Starting Circuits. For different lamp sizes and 
designs, the breakdown voltage may vary from 500 
to 1,500 volts or more. To provide consistently re¬ 
liable starting, the high-voltage starting supply 
should have sufficient current capacity to initiate a 
small concentrated arc and not merely to establish 
a high-voltage spark discharge, since the starting 
current must cause the arc voltage to fall to a value 
equal to or less than the open-circuit voltage of the 
operating supply. Three methods which have been 
employed to start the arc are described in the fol¬ 
lowing paragraphs. 

In the first method, the lamp is connected to the 
d-c operating supply in series with the proper bal¬ 
last resistance, and the output from a high-fre¬ 
quency spark coil is applied to the circuit. This 


method is often convenient in the laboratory, but is 
not considered satisfactory for general use, since 
the radio-frequency characteristics of the associated 
wiring may determine whether a given lamp will 
fire, and also the open-circuit voltage required for 
the d-c operating supply is likely to be higher than 
for other starting methods. 

In the second method, the transient inductive 
voltage pulse obtained by interrupting the current 
through a choke in series with the lamp is used to 
break down the arc gap. The lamp then continues 
to operate from the same power source which ener¬ 
gized the starting circuit. The principal advantage 
of this method lies in the fact that relays may be 
used to make the circuit automatically self-starting 
and to repeat the starting cycle automatically if the 
lamp is accidentally extinguished. However, this 
method is completely satisfactory only for the 2- 
watt lamp; for larger lamps the third method is 
more efficient and reliable. 

The third method utilizes more expensive com¬ 
ponents and power requirements, but at the same 
time provides more positive and generally satisfac¬ 
tory starting. It consists essentially of a poorly 
regulated high-voltage rectifier having an open-cir¬ 
cuit voltage of 1,000 to 2,500 volts. 




119V AC Tl WU WM232-H C2 I MFD 

T2 ■ WM232-F SI START SWITCH 

LI « WM232-C2 S2 OFF-ON SWITCH 
L2 STARTING RELAY SR FULL-WAVE SELENIUM 
RECTIFIER 

Figure 11. Power supply for 25-watt concentrated-arc 
lamp. 

A representative power supply circuit for starting 
a 25-watt lamp and operating it from a selenium 
rectifier is shown in Figure 11. After the filament 
of the starting rectifier tube VI has been heated by 


IFE S TOTCfE n 



















































































































GASEOUS DISCHARGE LAMPS 


31 


closing the main switch S2, the starting switch SI is 
closed, applying plate voltage to the rectifier tube 
and rectified high voltage directly to the lamp ter¬ 
minals. The arc gap instantly breaks down, and a 
current of 100 to 200 milliamperes flows through the 
lamp and the coil of the starting relay L2. Operation 
of the relay connects the output of the selenium 
rectifier to the lamp in parallel with the high voltage 
supply and the lamp current rises to its normal 
value of 1.3 amperes. Since the selenium rectifier 
current also flows through the relay coil, the relay 
contact remains closed and the lamp continues to 
operate after the starting switch is opened. 

Fully automatic methods for starting the lamp 
and for switching over to the operating circuit may 
be added when necessary. The added cost and com¬ 
plexity of this measure are frequently not justifiable 
if the lamp is to be used simply as a source of 
steady radiation, but may be essential if it is to be 
electrically modulated as the radiation source for a 
communication system. 

Operating Circuits. The concentrated-arc lamps 
can be operated from any d-c source capable of 
delivering a sufficiently smooth current at the re¬ 
quired voltage. Except for the 2-watt lamp, an 
operating potential difference of about 16 to 24 volts 
at the lamp terminals is ordinarily adequate (see 
Figure 9), to which must be added an allowance of 
about 50 per cent for the potential drop across the 
series ballast impedance. The required power may 
be obtained from storage batteries, a motor-gen¬ 
erator set, or a-c power with an electron-tube or a 
selenium rectifier. The final choice of power supply 
in each case will, of course, depend on the require¬ 
ments of the particular application and the available 
facilities. The selenium bridge-type rectifier is most 
commonly used in the power supply units con¬ 
structed by the Western Union Telegraph Company. 
A typical circuit for such a unit is shown in Fig¬ 
ure 11. For the 2-watt lamp, a special a-c operated 
power supply has been devised 23a in which very 
satisfactory starting and operating are achieved 
with only one transformer and one rectifier tube. 
This circuit also provides fully automatic starting 
and switch-over to continuous operation. 

Because the arc acts as a negative resistance it is 
essential for stable operation of the lamp that 
enough positive impedance be included in the oper¬ 
ating circuit to neutralize this characteristic. In 
general, a ballast resistance is sufficient if the volt¬ 


age drop across it is about one-half of that across 
the arc. A rheostat with provision for manual con¬ 
trol may be satisfactory, or a constant-current bal¬ 
last tube of the type frequently used in electronic 
circuits may be more convenient, for example, a tube 
such as those manufactured by the Amperite Com¬ 
pany. With a ballast tube of this type it is best to 
keep other impedances in the circuit as low as pos¬ 
sible in order to secure maximum effectiveness of 
current regulation. The degree of regulation required 
depends, of course, on the type of power supply used 
and the other conditions of each specific application. 

By means of specially constructed experimental 
arc lamps it has been demonstrated that the radia¬ 
tion characteristics of the cathode spot in the d-c 
operated concentrated-arc lamp can be duplicated 
by raising the cathode to incandescence either by 
joule heating or by electron bombardment. Many 
lamps of the 10-watt and larger sizes can also be 
operated directly from a low-frequency a-c source, 
the lamps acting as self-rectifiers. Ballast imped¬ 
ance should also be included in the circuit for this 
type of operation. 

Static Radiation Characteristics 

As shown in Table 5, the maximum brightness of 
the cathode spot in the standard concentrated-arc 
lamps is from about 40 to 100 candles per square 
millimeter. These values are about one-half as great 
as for the crater of a small d-c operated plain car¬ 
bon arc, and from three to ten times as great as for 
incandescent tungsten filaments near 3000 K. Up 
to the current value at which the cathode spot com¬ 
pletely fills the zirconium oxide surface, the size of 
the spot rather than its brightness is principally 
affected by changes in the arc current. An increase 
in brightness will result from still higher current, 
but the life of the lamp will be shortened. At current 
values very much less than normal the spot becomes 
variable in size and position. The maximum bright¬ 
ness is always found near the center of the spot. The 
lamps are not sufficiently stable either in total in¬ 
tensity or brightness distribution over the cathode 
spot to constitute a satisfactory substitute for in¬ 
candescent tungsten lamps as photometric standards. 

Intensity measured as a function of angle from 
the normal to the cathode has the distribution ex¬ 
pected for a plane disk which radiates in accordance 
with the cosine law. 

During the first few hours of operation the maxi- 






32 


NEAR INFRARED SOURCES 


mum brilliance of the cathode spot increases to 
about 130 per cent of the initial value. Thereafter 
the brilliance, candlepower, and cathode spot diam¬ 
eter all show a gradual decrease throughout the 
remaining life of the lamp. The life is said to be 
terminated when the arc is no longer concentrated, 



Figure 12. Average static candlepower characteris¬ 
tics of aged concentrated-arc lamps. Arrows indicate 
normal operating current. 


that is, when the cathode spot is dull and extended 
rather than white and concentrated or when the 
lamp can no longer be started with the standard 
equipment ordinarily used for this purpose. These 
conditions are usually caused by shrinkage or loss 
through vaporization of the cathode filling material. 
The average life is 175 hours for 2-watt lamps and 
900 hours for 10-watt lamps. Individual lamps in 
the 25-watt and 100-watt sizes have had lifetimes 
up to 5,000 hours when not modulated, but sufficient 
data are not available to indicate an average life for 
these sizes. The life of a lamp operated on direct 
current without modulation is apparently no differ¬ 
ent for intermittent start-stop operation than for 
continuous operation. 

Intensity versus Current and Power. As shown in 
Figure 12, the relationship between candlepower and 
current is almost linear over a very wide range of 
current for each lamp size. The major part of the 
candlepower change is due to an automatic change 
in the diameter of the cathode spot as the current 
is varied. 

The efficiency of concentrated-arc lamps, meas¬ 
ured in candlepower per watt input to the lamp (not 
including the power expended in the ballast imped¬ 
ance) as a function of current, is shown in Figure 13. 
The higher efficiency values for the lamps of larger 


size may be due to relatively lower heat losses from 
the activated radiating surface to the cathode sup¬ 
porting structures. The figures are, on the average, 
only about one-half as great as for incandescent 
tungsten lamps. Moreover, because of the spatial 
distribution characteristics of a disk-type radiator. 



Figure 13. Average static luminous efficiency of aged 
concentrated-arc lamps. Arrows indicate normal oper¬ 
ating current 


the total flux from a concentrated-arc lamp is only 
about one-third of that from a tungsten lamp of 
equal hololuminous intensity, or about one-sixth of 
that from a tungsten lamp operated at equal power. 

Spectral Distribution. A typical spectral distribu¬ 
tion curve for the wavelength region from 0.3 to 
1.0 p is shown in Figure 14. The spectrum consists 



Figure 14. Static spectral distribution of radiation 
from concentrated-arc lamps. 


essentially of a continuum upon which are super¬ 
posed spectral lines of zirconium and of the filling 
gas. The peak of the radiation curve occurs near 
1.0 p and the curve resembles that of a black body 












































































































































































GASEOUS DISCHARGE LAMPS 


33 


near 2800 K, lying somewhat above the black body 
curve on both sides of the peak. The spectral dis¬ 
tribution in the near infrared is thus quite similar 
to that for an incandescent tungsten lamp, and be¬ 
cause of the large amount of radiation in the visible 
region a relatively dense filter is required to provide 
the security which is essential for applications in 
military signaling and communication systems. It 
has been estimated that more than 90 per cent of 
the total radiant energy originates from the incan¬ 
descent cathode spot, in contrast to the relatively 
small amount which originates in the gaseous arc 
column. 

Dynamic Electrical Characteristics 

When an a-c voltage wave is impressed on the 
terminals of a concentrated-arc lamp through which 
direct current is flowing, the resulting a-c current 
wave lags behind the applied a-c potential wave by 
an angle whose magnitude depends on the impressed 
frequency. The modulated radiation wave in turn 
lags behind the current wave, but by an amount 
which is generally much smaller than the current 
voltage lag. Distortion of the radiation wave is not 
large for frequencies in the audio range, so that the 
quality of voice reception in systems using this 
source is determined principally by factors other 
than arc characteristics. 

The dynamic electrical characteristics may be 
summarized as follows: 

1. The equivalent internal impedance of the lamp 
is composed of a variable resistance and an induc¬ 
tive reactance. 

2. The resistive component decreases with a de¬ 
crease in the unmodulated direct-current operating 
value, becoming negative at low d-c values. The 
magnitude and sign of the resistive component also 
depend on the frequency of the modulating poten¬ 
tial. 

3. The inductive reactance component increases 
with a decrease in the direct-current operating 
value. The equivalent inductance is almost inversely 
proportional to modulation frequency, the inductive 
reactance being essentially independent of fre¬ 
quency. 

4. The impedance is definitely nonlinear over the 
medium range of current modulation ordinarily em¬ 
ployed. 

5. The modulated radiation is almost directly 
proportional to the modulated current within the 


usual range of per cent current modulation, but lags 
behind it by a small phase angle which varies so 
slowly with frequency as to be of little importance 
in communication systems. 

Typical curves showing the dependence of the 
electrical characteristics on frequency for lamps oper¬ 
ated at rated d-c values and at 50 per cent current 
modulation are given in Figure 15, which refers 
specifically to the 10-watt lamp. The effective im¬ 
pedance of the lamps varies from a few hundred 
ohms in the 2-watt size to only a few ohms in the 



Figure 15. Impedance and phase characteristics of 
10-watt concentrated-arc lamps. 


100-watt and larger sizes. It is of interest to note 
that a minimum occurs in the curve of impedance 
versus frequency; the frequency at which it occurs 
becomes lower as the size of the lamp is increased. 
It is thought that the lamp may have negative re¬ 
sistance at some much higher frequency range also, 
since self-sustained radio-frequency oscillations 
have sometimes been found to exist in the lamp cir¬ 
cuits. 

Modulating Circuits. A sine-wave modulating 
current will produce essentially a sine wave of radia¬ 
tion from the concentrated-arc lamp, the only dis¬ 
tortion being the relatively unimportant phase 
distortion. A sine wave of voltage will not produce 
this result, however, since the volt-ampere charac¬ 
teristic is nonlinear in both amplitude and phase. 
Moreover, the arc is limited in the amount of current 
it can pass because of the limitations of heat dissi¬ 
pation and current saturation. When overdriven it 
will to some extent rectify the applied signal, caus¬ 
ing an additional d-c component to flow and dimin¬ 
ishing slightly the d-c terminal voltage. 

When stability of operation and low distortion 
























































34 


NEAR INFRARED SOURCES 


are important, as for use in a communication sys¬ 
tem, the following general requirements should be 
observed in the operating and modulating circuits. 

1. The effective impedance in series with the lamp, 
including the effect of other circuits in parallel with 
the lamp circuit, should always be greater than any 
instantaneous negative impedance presented by the 
lamp. 

2. The best linearity between the electric input 
and the modulated radiant output is obtained -when 
the lamp is modulated by a constant-current gen¬ 
erator. Such a generator, having infinite internal 
impedance, also provides a maximum of stability. 

3. The electric modulation source should be ca¬ 
pable of supplying the peak instantaneous modula¬ 
tion power over the desired frequency band to a 
load having distinctly nonlinear characteristics and 
yet not be overloaded itself. 

4. The possibility of extinguishing the arc by 
overmodulation can be minimized by removing the 
low frequencies for which the possible period of 
overmodulation is equal to or greater than the time 
in which the arc may be extinguished, or it can be 
completely eliminated by limiting the amplitude of 
the signal. 

5. The d-c operating supply should be sufficiently 
smooth to prevent the introduction of undesired 
modulation components or frequencies. 

The impedance of the 2-watt lamp is sufficiently 
high so that it can be successfully modulated in the 
plate circuit of a vacuum tube, the modulating volt¬ 
age being applied to the grid. The larger lamps do 
not permit modulation by this method. 

The most economical and generally satisfactory 
method of modulating the lamp is to apply the 
modulating current directly through the lamp and 
not through the ballast resistance and other circuit 


R-1 R-2 



Figure 16 . Schematic method of modulation. 

components. This basic method is shown schemati¬ 
cally in Figure 16; its successful application depends 
on keeping the shunt impedances of the starting and 
operating circuits high as compared with the imped¬ 
ance presented by the lamp. In practice it is neces¬ 


sary to modulate lamps larger than the 2-watt size 
through an impedance-matching power transformer 
and a series blocking condenser. The actual power 
required to modulate the lamp may vary from 50 
to 100 per cent of its nonnal d-c power rating, de¬ 
pending on the size of the lamp and the operating 
conditions which it is desired to meet. 

Characteristics as a Source of Modulated 
Radiation 

Radiation from the concentrated-arc lamp ap¬ 
pears to consist of the following three parts which 
are listed according to the total radiation emitted: 
continuous radiation from the incandescent cathode 
spot, line radiation from the excited gas and vapor, 
and some continuous radiation, at least in the 
shorter wavelength visible regions, originating in 
the excited gas and vapor. As would be expected on 
this basis it is found that the modulation character¬ 
istics of the emitted radiation depend upon its wave¬ 
length, upon the modulating frequency, and upon 
whether average characteristics of the entire radia¬ 
tion emitted by the lamp are investigated or only 
the characteristics of that portion of the radiation 
which originates in the arc stream or at a selected 
region of the cathode. 

A rather detailed experimental investigation of 
the modulation characteristics has been made from 
which the following conclusions were drawn. 244 A 
part of the radiation, most pronounced in the blue 
and ultraviolet regions of the spectrum, responds 
almost instantaneously to changes in the arc current. 
A considerably larger part, whose relative value also 
varies with wavelength, follows the arc current with 
a time lag of the order of 10~ 4 second. A third part, 
especially prominent in the infrared region and be¬ 
lieved to arise primarily as thermal radiation from 
a sharply defined incandescent spot on the cathode 
surface having a relatively low true temperature of 
the order of 2000 K, is not subject to appreciable 
modulation over the useful range of audio frequen¬ 
cies. The relative amount of infrared radiation 
which cannot be modulated at useful frequencies is 
thought to depend critically on the thermal insu¬ 
lation of the material forming the thin, activated 
surface of the cathode, and on the composition and 
behavior of this material under bombardment by 
ions from the arc column. Presumably this radiation 
can be modulated only by changing the temperature 
of the cathode surface, the thermal inertia of which 















GASEOUS DISCHARGE LAMPS 


35 


is too great to permit appreciable modulation within 
the audio-frequency range. 

The spectral distribution of modulated radiation 
from the concentrated-arc lamp at a modulation 
frequency of 1,000 cycles per second is shown in 
Figure 17. The relative dependence of the modulated 
intensity upon wavelength is illustrated by com¬ 
paring the curve of Figure 17 with the static spec¬ 
tral distribution curve of Figure 14. The data for 
Figure 17 were obtained by using an a-c amplifier 



Figure 17. Spectral distribution of 1000-cycle modu¬ 
lated radiation from concentrated-arc lamps. 


following the photoelectric radiation detector so that 
the unmodulated radiation at each wavelength was 
not measured. A series of similar curves obtained 
for other modulation frequencies show that as the 
modulation frequency is increased the amplitude 
of the continuum, which originates primarily from 
the cathode, decreases by a much larger factor than 
the amplitude of the spectral lines, which originate 
in the cathode glow region. A high modulation ratio 
for the spectral lines exists throughout the entire 
audio-frequency range, the upper frequency limit 
for successful modulation presumably being deter¬ 
mined by the deionization time of the gas. 

The dependence of the modulation ratio upon fre¬ 
quency and upon spectral region is shown for one 
lamp in Figure 18. The ultraviolet curve was ob¬ 
tained by using a combination filter (Corning 430 
with Corning 584) which has a transmission peak at 
about 3600 Angstrom units, in conjunction with 
a photomultiplier tube (RCA type 931) having an 
ultraviolet-sensitive antimony cathode. The other 
curves represent the response of an infrared-sensitive 
cesium-cathode phototube to the modulated com¬ 


ponent of the radiation transmitted by the following 
filters: Corning 430 (blue), Corning 241 (red), 
and Polaroid XR7X (infrared). 



Figure 18. Modulation ratio versus frequency for 
different wavelength regions of concentrated-arc radi¬ 
ation. 




Figure 19. Modulation characteristics of 10-watt con¬ 
centrated-arc lamps with reference to a cesium-surfaee 
detector. 


The modulation characteristics of the 10-watt 
concentrated-arc lamp, as an example, are shown on 
a somewhat different basis in Figure 19. These 


































































































































36 


NEAR INFRARED SOURCES 


curves were obtained by using the entire radiation 
from a lamp with no filter interposed and with an 
infrared-sensitized cesium-cathode phototube as the 
detector. The per cent modulation of the emitted 
radiation at 100 per cent current modulation is, by 
definition, the modulation ratio. The per cent modu¬ 
lation of the radiation shows a decrease as the lamp 
size is increased, but, because of the greater quan¬ 
tity of radiation emitted by the larger lamps, the 
total modulated radiant energy is considerably 
greater for the large than for the small lamps. The 
lower graph of Figure 19 shows the percentage of 
the modulated radiation having the second-har¬ 
monic frequency. The distortion of the modulated 
radiation which results from the lack of an accu¬ 
rately linear relationship between lamp current and 
emitted radiant energy has been found to consist 
largely of this component, and increases with an 
increase in modulating frequency, per cent current 
modulation, and lamp size. By limiting the current 
modulation, the distortion can ordinarily be lim¬ 
ited to a range within the permissible limits for 
usual audio-frequency amplifier practice so that it 
does not perceptibly affect the overall fidelity of a 
communication system. 

It is seen, therefore, that the per cent modulation 
of the emitted radiation may vary over a rather 
wide range, depending upon the exact conditions in 
which the lamp is used. For certain special applica¬ 
tions in which only a small amount of radiant flux 
having the highest possible per cent modulation is 
desired, a lens and slit arrangement may be used to 
isolate that portion of the radiation which originates 
from the cathode glow or from the center of the 
cathode spot. The per cent modulation under some 
conditions may be 50 per cent greater for lamps 
filled with krypton than for lamps filled with argon. 
Very promising results have also been obtained in 
experimental lamps filled with gas at a pressure of 
10 to 15 atmospheres instead of the usual one at¬ 
mosphere and by the use of especially designed ex¬ 
perimental cathodes which will not be described 
here. Experimental variations in anode construction 
have also been investigated. Although some of these 
measures result in lamps of increased brilliance and 
higher radiant efficiency or modulation efficiency, 
they have not yet been successfully developed to a 
point suitable for application in large-scale pro¬ 
duction. 

Life of Lamps when Modulated. When the current 


through a lamp is modulated, the life of the lamp 
may be considerably reduced, probably due to the 
loss of zirconium vapor from the cathode glow 
region during the portion of the modulation cycle 
in which the cathode is least negative. The amount 
by which the lamp life is reduced apparently de¬ 
pends upon the average value of the direct current 
and upon the waveform, frequency, and amplitude 
of the modulating current, particularly if the po¬ 
larity applied to the lamp is actually reversed dur¬ 
ing any portion of the modulation cycle. The exact 
nature of the dependence of lamp life upon these 
factors is not known, but under some conditions the 
life has apparently been reduced by as much as 90 
per cent. 

Applications in Wide-Beam Optical Systems 

Because of its small diameter and high brilliance, 
the cathode of a concentrated-arc lamp constitutes 
an ideal source for projecting a narrow beam of ra¬ 
diation which can be electrically modulated for 
voice or code communication purposes. When the 
cathode is located at the focal point of the projec¬ 
tion system, the angular spread of the beam is ap¬ 
proximately equal to the angle subtended by the 
cathode spot at the vertex of the reflector or the 
optical center of the lens or equivalent lens system. 
Thus for a system having a focal length of 8 inches, 
the value of the cathode spot diameter shown in 
Table 5 for the 25-watt lamp corresponds to a com¬ 
puted angular beam spread of only about 0.2 degree. 

Although the lamps have been used in narrow- 
angle projection systems by other agencies, their 
principal application within Section 16.4 occurred 
in the early stages of development of the type E in¬ 
frared voice and code communication system, 24 for 
which a beamwidth variable between 15 and 40 
degrees was originally requested (see Chapter 4). 
The problem of redistributing radiation from what 
is essentially a point source into a fairly uniform 
beam having a spread of this order of magnitude is 
rather specialized and has been treated in detail 
elsewhere. 26 

Some of the problems associated with the pro¬ 
duction of a wide-angle beam from a very small 
source are the following: (1) since the total flux 
available from the source is limited by its small size 
no matter how high its brilliance, and since for any 
given total flux available from the source the in¬ 
tensity available at a receiver will vary inversely 


jt^eeiETr' 




GASEOUS DISCHARGE LAMPS 


37 


as the solid angle into which it is spread, it is essen¬ 
tial that the greatest possible fraction of the flux 
emitted by the source be collimated within the 
specified angular limits of the beam and that the 
source and its mountings obscure the smallest pos¬ 
sible portion of the collimated beam; (2) aberra¬ 
tions in classical idealized optical systems utilizing, 
for example, surfaces of revolution having conic sec¬ 
tions, give rise to nonuniformities of intensity across 
the beam; (3) the small size of the source demands 
extraordinarily high precision in the preparation of 
the reflecting or refracting surfaces since an irregu¬ 
larity in the beam pattern caused by an imperfec¬ 
tion in the optical surface cannot be smoothed over 
by flux originating from an adjacent area of the 
source, as is the case with an extended source. 

The possibilities of various types of optical sys¬ 
tems have been investigated both theoretically and 
experimentally. With the 100-watt concentrated-arc 
lamp as the source it has been found possible to pro¬ 
duce a rectangularly shaped beam having angular 
dimensions 18x8 degrees and with a sufficiently 
uniform intensity distribution to be satisfactory for 
communication and signaling systems. In the most 
compact and efficient system devised for this pur¬ 
pose (see Chapter 4) the source is mounted, facing 
in the direction of the emergent beam, at the focus 
of a spun Alzak parabolic reflector having a focal 
length of 0.6 inch, 6 inches in diameter and 7 inches 
deep. An //1.2 plano-convex lens is used to colli¬ 
mate radiation that would otherwise pass uncolli¬ 
mated out the front opening of the reflector. Over 
the front of the reflector is placed an 18x8-degree 
spread lens of a type which is available, for exam¬ 
ple, from the Holophane Company, Newark, Ohio. 
The multiple-facet construction of the spread lens 
serves not only to give the desired divergence but 
also smooths out the irregularities which would 
otherwise exist due to the surface imperfections in 
commercially available optical elements. More com¬ 
plete details of this and related systems and a gen¬ 
eral discussion of source and transmitter optics and 
of other factors in relation to the performance char¬ 
acteristics of complete communication systems are 
given in Chapter 4 and elsewhere. 245 ’ 27 

The beam candlepower of the 18x8-degree trans¬ 
mitter described above, using the 100-watt concen¬ 
trated arc as the source, is approximately 5,000 
equivalent holocandles with reference to a cesium- 
surface vacuum phototube. 


Infrared Filters for Concentrated-Arc Lamps 

Since the spectral energy distribution of radiation 
from concentrated-arc lamps consists principally of 
a continuum with a distribution similar to that for 
the radiation from incandescent tungsten lamps, the 
infrared filters required to give them sufficient visual 
security for military purposes are quite similar to 
those required for use with tungsten lamp sources. 
Basic problems and general considerations pertain¬ 
ing to infrared filters are outlined in Chapter 2, 
while those selected for use in specific near infrared 
systems are indicated in Chapters 4, 5, 6, and 7. 

The ehT values of filters measured with reference 
to the modulated radiation from electrically modu¬ 
lated concentrated-arc lamps are considerably lower 
than the values for the same filters with reference 
to radiation from unmodulated or mechanically 
modulated tungsten lamps for two reasons. First, 
the near infrared component of concentrated-arc 
radiation is relatively smaller than for incandescent 
tungsten radiation, the arc radiation being richer in 
energy at visible wavelengths. Second, as shown by 
Figure 18, the modulation ratio at different wave¬ 
length bands decreases with increasing wavelength, 
so that an infrared filter apparently discriminates 
more strongly against the modulated infrared com¬ 
ponents than its spectral transmission curve, or ex¬ 
perimental tests made with an unmodulated lamp, 
would indicate. 

The ehT values of three representative infrared 
filters with reference to radiation from an arc fully 
modulated at 1,500 cycles per second, as measured 
by two different infrared detectors, are shown in 
Table 6. Also included for each of these filters is the 
calculated visual range of a transmitter having an 
intensity of about 5,000 holocandles in the absence 
of a filter, such as the one described in the preced¬ 
ing section. Corresponding ehT° values for the 
Polaroid XR3X41 filter, with reference to radiation 
from an incandescent tungsten source at a color 
temperature of 2848 K, are about 27 per cent for 
the cesium-cathode phototube and about 45 per cent 
for the thallous sulfide photoconductive cell (see 
Chapter 3). 

Discussion 

Concentrated-arc lamps in the four standard sizes 
which are now commercially available constitute 
an interesting and potentially valuable source of 



38 


NEAR INFRARED SOURCES 


near ultraviolet, visible, and near infrared radiation. 
The lamps have been successfully used for a num¬ 
ber of applications requiring a high-intensity, con¬ 
centrated radiation source. The feasibility of con¬ 
structing lamps up to 1,000-watt ratings using 
water-cooled electrodes has also been demonstrated, 
and experiments initiated before the conclusion of 
NDRC sponsorship indicated the possibility of 
further improvements in radiation efficiency and 
electrical modulation characteristics. 


Table 6. Effective holotransmission of various filters 
and calculated visual range of a 5,000-holocandle 
transmitter using the concentrated-arc lamp. 



ehT for modulated radi¬ 
ation component from 
arc fully modulated at 
1,500 cycles 

Gas-filled Thallous 

Calculated 

visual 


Cb photo¬ 

sulfide 

range 

Filter 

tube 

cell 

(yards) 


Wratten 87 

0.35 

0.55 

8,000 

Polaroid XR3X41 

0.06 

0.16 

70 

Polaroid XR7X25 

0.03 

0.14 

35 


A great many scientific questions regarding the 
operation of concentrated-arc lamps and the expla¬ 
nation of their characteristics remain unanswered 
and should be explored in greater detail. For ex¬ 
ample, estimates for the temperature of the cathode 
surface based on (1) the spectral distribution of the 
radiated energy, (2) Stefan’s law and the total 
energy radiated, and (3) the electron emission 
needed to maintain the arc using the work function 
of metallic zirconium which might be produced by 
reduction of the oxide, differ by nearly 1000 K. 
Other questions immediately arise, such as whether 
the thin activated layer at the cathode spot is 
molten or solid during operation; the origin of the 
continuum, its spectral distribution, and its relation¬ 
ship to other continua; the possible relation between 
the semiconducting properties of the cathode surface 
and the time lag in the emitted radiation; and the 
frequently contradictory results of measurements 
made in different laboratories on lamp life, modula¬ 
tion ratio, spectral distribution of modulated and 
unmodulated radiation, etc. 

Although the possibility of using concentrated- 
arc lamps in wide-angle modulated-beam communi¬ 
cation systems was successfully demonstrated, a 


less concentrated type of source, such as the cesium- 
vapor lamp described in the following section, is 
generally to be preferred for this purpose. 

1,3,3 The Cesium-Vapor Lamp 

The cesium-vapor lamp was developed by the 
Lamp Development Laboratories of the Westing- 
house Electric Corporation under Navy contract 
and was made available in 1944 for NDRC use as 
a source of near infrared radiation. Two different 
sizes of this lamp have been standardized in coop¬ 
eration with Northwestern University under Con¬ 
tract OEMsr-990 for use as the source in infrared 
communication and signaling systems 24 > 28 developed 
by that contract (see Chapter 4). It is understood 
that the development of additional sizes of this 
lamp for other applications has been initiated. 

The radiation emitted by this lamp originates in 
the column of a low-pressure arc discharge in 
cesium vapor. About 20 per cent of the input power 
is radiated in the two cesium resonance lines at 
wavelengths of approximately 0.85 and 0.89 \i. Since 
most of the radiation originates in the arc column, a 
much higher modulation ratio may be achieved than 
is possible for a concentrated-arc lamp. The visual 
security required for military applications may be 
achieved with a much less dense filter than is needed 
for sources which have a high-intensity continuous 
spectrum. The electrical characteristics and methods 
of operation are very similar to those described in 
Section 1.3.2 for concentrated-arc lamps and will be 
described largely by reference to that section. 

Lamp Design and Construction 

The details of construction of the 90-watt type 
CL-2 cesium-vapor lamp are shown in Figure 20. 
The same general type of construction is used in 
other sizes of this lamp, for example, the 50-watt 
lamp employed in the aircraft communication sys¬ 
tems 28 described in Chapter 4. The inner bulb is 
made of heat-resistant glass (Corning No. 705) 
coated with a special alkali-resisting glaze to pre¬ 
vent corrosive attack by the cesium vapor. It con¬ 
tains a small amount of cesium metal and is filled 
with neon or other rare gas at a low pressure. The 
electrodes consist of coiled tungsten filaments, 
spaced about three inches apart, by means of which 
the lamp is preheated for 1 to 3 minutes and the 
cesium metal is vaporized. The cathode filament is 







GASEOUS DISCHARGE LAMPS 


39 


coated with a barium-strontium oxide mixture to 
afford the copious emission of electrons needed to 
maintain the arc. The outer envelope is a T-16 bulb, 
evacuated to reduce heat losses and mounted in a 
standard 4-pin base. 



BASE-END VIEW 

Figure 20. The Westinghouse type CL-2 cesium- 
vapor lamp. 

Electrical Characteristics and Operating 
Circuits 

Since the arc is maintained in a metallic vapor, 
the electrical and radiative characteristics are both 
complicated by thermal time-lag effects in that the 
instantaneous characteristics are dependent on the 
immediately previous thermal history of the lamp. 
The 50-watt lamp, which has an electrode spacing of 

1.5 to 2 inches, is normally operated at about 12 
volts and 4 amperes direct current. The type CL-2 
lamp consumes about 90 watts when operated at 

5.5 amperes direct current. The static volt-ampere 
curve is essentially flat from 1 to 8 amperes. The 


average voltage at the lamp terminals over this 
range of current may vary between 11 and 25 volts, 
not only for different lamps but also for a given 
lamp over its lifetime. The circuit requirements for 
stable d-c operation are essentially the same as for 
concentrated-arc lamps. The static radiant intensity 
of the lamp is approximately proportional to the 
direct-current value when thermal equilibrium is at¬ 
tained. Presumably the operating temperature, elec¬ 
tric power input, and radiant output might be 
increased by continuously heating the starting fila¬ 
ments. However, continuous operation of the fila¬ 
ments from the normal 2.5-volt a-c supply was 
found to introduce a 120-cycle ripple in the radia¬ 
tion output large enough to be objectionable in a 
communication system. 

The dynamic internal impedance of the 90-watt 
lamp at 1,500 cycles per second is of the order of 

1.6 ohms when the lamp is operated at 5.5 amperes 
direct current. It consists principally of a resistive 
component, plus an inductive reactance of the order 
of 0.4 ohm. The variations of impedance with fre¬ 
quency and with the magnitude of the direct current 
are identical in nature with those for concentrated- 
arc lamps, but, in many respects, the electrical be¬ 
havior is considerably superior. The cesium lamp is 
much more stable and can be operated up to 100 per 
cent current modulation without danger of being 
extinguished. Moreover, the dynamic voltage-cur- 
rent relation is very nearly linear over a much larger 
current modulation amplitude than for concen¬ 
trated-arc lamps, and the radiant energy emitted is 
essentially proportional to and in phase with the 
arc current over the entire audio-frequency commu¬ 
nication band. A considerable improvement in the 
tone quality and intelligibility of voice communica¬ 
tion therefore results from the use of this lamp. 

Because of the long filament and lead structure of 
the lamp a considerable variation in its performance 
may result from different modes of connecting the 
modulating circuit to the electrodes. The most uni¬ 
form performance from different lamps is obtained 
by connecting the modulating current to the center 
taps of the filament transformers. The principles of 
operation and the characteristic features of the 
modulating circuits required for optimum perform¬ 
ance are identical with those already described for 
the concentrated-arc lamp. 

Starting Circuits. The starting of the cesium-vapor 
lamp is affected by several factors. The cesium 





































40 


NEAR INFRARED SOURCES 


vapor condenses on the inside of the bulb when the 
lamp is inoperative so that, because of time-lag 
effects, its vapor pressure depends much more crit¬ 
ically upon the recent thermal history of the lamp 
than upon the pressure of the inert filling gas. The 
condensed cesium may, under certain circumstances, 
partially or completely short-circuit the arc gap for 
starting purposes. Some of the cesium originally 
available is gradually lost by combination or ab¬ 
sorption in the bulb, and the condition of the start¬ 
ing filaments with reference to electron emissive 
properties is also important. 

It is therefore impossible to enumerate the opti¬ 
mum starting conditions for all circumstances. 
However, it is found that reliable starting is attained 
in most applications by preheating the filaments for 
one minute and then applying 400 volts alternating 
current or 500 volts direct current. The starting arc 
current should lie preferably between 0.5 and 1.0 
ampere. The filament heating source is disconnected 
once the arc is established on the operating power 
supply. The switch-over requirements and the gen¬ 
eral types of circuits which can be used for starting 
and operating the lamps are quite similar to those 
already described for use with the concentrated-arc 
lamp. 

Radiation Characteristics 

Due to the low intensity of the spectral lines 
emitted in the visible region, the visual intensity of 
the bare 90-watt cesium lamp is only about 5 can¬ 
dles. With reference to a cesium-surface detector, 
however, the intensity is from about 90 to 150 equiv¬ 
alent holocandles for the 90-watt lamp, and about 
70 holocandles for the 50-watt lamp. The visual out¬ 
put apparently varies at some power of the lamp 
current greater than unity, so that the luminous 
intensity of the lamp increases during modulation 
even though the average current remains constant. 
This visual effect is entirely eliminated if the lamp 
is viewed through a relatively light infrared filter 
such as Wratten 87. The spatial intensity distribu¬ 
tion is essentially that to be expected from the sine 
distribution law for a linear source, and the total 
flux emitted is about three times as great as that 
from a plane disk source of equivalent intensity, 
such as a concentrated-arc lamp, for which the co¬ 
sine intensity distribution law is followed. 

Being concentrated principally in the two cesium 
resonance lines at an average wavelength of about 


0.87 p, the useful infrared radiation from the lamp 
is essentially monochromatic. The ehT values of 
filters for cesium lamp radiation are therefore very 
close to the spectral transmission values of the filters 
at this wavelength and are almost independent of 
whether a cesium-cathode phototube, a thallous sul¬ 
fide photoconductive cell, or some other type of in¬ 
frared photodetector is used. 

The useful near infrared radiation originates ex¬ 
clusively in the arc column and has a very small 
time lag behind the modulating current. A high mod¬ 
ulation ratio is therefore to be expected; its average 
value is found to be about 0.92 from 200 to 5,000 
cycles per second, decreasing at higher frequencies 
to a value of about 0.70 at 10,000 cycles. Since the 
useful modulated infrared radiation is almost mono¬ 
chromatic, the modulation ratio is essentially inde¬ 
pendent of whether or not the source is covered with 
a filter. 

After a few minutes of operation to permit stabili¬ 
zation, the arc becomes a fairly well-defined cylin¬ 
drical column, about one-half inch in diameter, 
which may be somewhat bowed due to thermal con¬ 
vection currents. After a few hours of operation a 
brownish stain begins to appear on the inner bulb, 
probably due to a reaction of the cesium with the 
glass. This may eventually reduce the visual inten¬ 
sity of the arc by a factor of three or more without 
appreciably affecting the intensity of the near infra¬ 
red radiation. The construction and operating char¬ 
acteristics of the lamp are similar to those of the 
familiar sodium-vapor lamp so widely used as an 
efficient, monochromatic source for visual illumina¬ 
tion. 

Spectral Energy Distribution. Since the lamp is 
normally operated with the power to the starting 
filaments cut off, a well-developed cesium line spec¬ 
trum constitutes its principal radiation in the vis¬ 
ible and near infrared regions. Additional energy is, 
of course, radiated in a thermal continuum having 
an energy distribution centered at much longer 
wavelengths, corresponding to the relatively low 
operating temperatures of the electrodes and the 
lamp envelope. The relative amount of energy ra¬ 
diated in various wavelength regions is shown in 
Table 7. Only two strong spectrum lines occur in the 
blue region, with the result that visual security is 
achieved with a filter of much lower opacity than 
would be required for a source of equivalent holo- 
candlepower having a continuous spectrum. The sec- 





GASEOUS DISCHARGE LAMPS 


41 


ond band listed includes the resonance lines, which 
contain most of the useful near infrared radiation. 
The experimental measurements were made with a 
vacuum thermocouple enclosed in a glass bulb, using 
a series of filters having different cutoff wavelengths. 
Although certain idealized assumptions were made 
in computing the values given in the table, the re¬ 
sults are certainly correct as to order of magnitude. 


Table 7. Distribution of energy radiated by the 
cesium arc. 


Wavelength region (microns) 

Per cent of total energy 

0.3 -0.78 

3 

0.78-0.9 

22 

0.9 -1.4 

5 

1.4 -3.5 

11 

Beyond 3.5 

59 (remainder) 


Life Test Results 

Accelerated life test data were obtained at North¬ 
western University with the lamps operated on a 
continuous start-stop cycle in equipment especially 
constructed for this purpose. The sequence of oper¬ 
ations during this test was as follows: 

1. Preheat the filament for 5 minutes. 

2. Start the lamp from a 480-volt, 0.5-ampere d-c 
starting circuit. 

3. Operate 6 minutes at 5.3 amperes direct cur¬ 
rent. 

4. Operate 49 minutes at 5.3 amperes direct cur¬ 
rent with 3.6 amperes (rms) modulating current at 
1,500 cycles per second superposed, corresponding to 
95 per cent current modulation. 

5. Allow the lamp to cool for one hour before re¬ 
peating the above cycle of operations. 

In this test the frequency of starting the lamp is 
greater than would occur in usual field practice, and 
the per cent current modulation is higher than aver¬ 
age speech modulation would require by a factor of 
two to four. In practical use the lamps are ordinarily 
operated at a stand-by current of one ampere in¬ 
stead of being allowed to cool completely before 
being restarted. The results of these tests should not 
be interpreted as applying directly to any other 
operating conditions, but they are indicative of the 
order of magnitude which can be expected for lamp 
life. 

Thirty-eight type CL-2 90-watt lamps were 
tested in this manner, including one lamp which was 


initially faulty. The life was found to vary from 
0 to 380 hours. The shortest life of a lamp success¬ 
fully started on the cycle was 2 hours. One half of 
all the lamps tested were expended at or before 65 
hours. 

Lamps may fail in one of two ways. (1) A crack 
may develop in the inner bulb, usually near the 
anode, with the result that the arc can no longer be 
started at all, or the leakage of gas from the inner 
bulb will permit an arc to start between the leads 
in the outer bulb and melt them. (2) If no crack 
develops, the lamp is considered to have failed when 
all the cesium is used up. The anode then becomes 
brilliantly incandescent and produces a discolored 
bulge in the inner bulb. Although a lamp may re¬ 
main operable with the residual gas for many hours 
after this occurs, its electrical and radiative charac¬ 
teristics are so altered that it is of little value. 

Applications in Optical Systems 

The total flux emitted by a linear cesium lamp 
source is about three times that from a plane disk 
source (such as a concentrated-arc lamp) having 
equivalent hololuminous intensity in directions per¬ 
pendicular to the line and plane, respectively. Never¬ 
theless the linear extent of the arc is so great that it 
is difficult to collimate a high percentage of the 
emitted flux within a beam width approximating 
20 degrees. Although a higher flux-gathering effi¬ 
ciency can generally be obtained with reflectors 
than with lenses, the efficient use of reflectors is 
rendered more difficult by the fact that the source 
is so large (2 to 3 inches long by % inch in diam¬ 
eter, and is, moreover, not transparent to its own 
radiation. 

For the 90-watt lamp, the most economical, effi¬ 
cient, compact, and simple optical system is that 
adopted for the type E transmitter 24c (see Chapter 
4). The lamp is mounted axially in an Alzak-sur- 
face aluminum reflector 7 inches deep, approxi¬ 
mately parabolic with an aperture of 14 inches and 
a focal length of 1.75 inches. The width of the re¬ 
sulting beam is somewhat smaller than was origi¬ 
nally desired, being approximately 13 degrees be¬ 
tween the points corresponding to one-half of the 
peak beam intensity; the peak hololuminous inten¬ 
sity of the transmitter measured at the center of the 
beam is 40 to 45 times that of a bare lamp, dropping 
to about ten times at 10 degrees from the axis of the 
beam. About 30 per cent of the emitted flux misses 










42 


NEAR INFRARED SOURCES 


the reflector entirely while another 20 per cent is lost 
at the reflecting surface. The remaining 50 per cent 
of the total flux is projected within a cone of 60 
degrees total angle. About 33 per cent of the total 
is projected within a 30-degree cone and about 22 
per cent within a 20-degree cone. When the desired 
beam width is only about 20 degrees, a somewhat 
shorter arc would result in a higher optical efficiency 
value for this type of mounting. 

In the communication systems for aircraft 283 
using the 50-watt lamp, a similar method of mount¬ 
ing was used in the plane-to-ground transmitter 
unit (see Chapter 4). In the interest of compactness 
a reflector 3 inches deep having a 7.25-inch diameter 
and %-i n ch focal length was used. The width of the 
beam between the points of one-half of the peak 
beam intensity is 16 degrees and the peak holo- 
luminous intensity of the transmitter measured at 
the center of the beam is 18 times that of a bare 
lamp. The peak beam intensity with reference to a 
cesium-surface detector when no filter is interposed 
is about 1,400 equivalent holocandles. Other trans¬ 
mitter units in which a still wider beam was desired 
were constructed by using combinations of plane 
mirrors, with a corresponding reduction in the peak 
beam intensity. Details of these units may be found 
in the references given above. 

Infrared Filters for the Cesium-Vapor Lamp 

The general characteristics of infrared transmit¬ 
ting filters are treated more fully in Chapter 2, while 
the operating characteristics achieved in communi¬ 
cation systems using certain specific filters with the 
cesium-vapor lamp are outlined in Chapter 4. The 
basic theoretical considerations from which the op¬ 
timum transmission characteristics of a filter may 
be predicted in attempting to meet specifications on 
visual security and operating range for a particular 
transmitter and receiver combination are given in 
detail elsewhere. 24 * 1 ’ 29 

Because the cesium-vapor lamp emits a line spec¬ 
trum rather than a continuum, the numerical rela¬ 
tion between its visual and holo intensity with refer¬ 
ence to a near infrared radiation detector is quite 
different from that for an incandescent tungsten 
source or a concentrated-arc lamp. This difference 
is necessarily reflected in the choice of an optimum 
filter for the two types of source. Since most of the 
radiation that is useful for infrared communication 
systems is concentrated at 0.85 and 0.89 p, it might 


appear that the ideal filter for the cesium lamp 
would transmit all of the radiation at these wave¬ 
lengths and none at the shorter wavelengths which 
contribute little to the communication range but 
much to the visual effect. Even with this supposedly 
ideal filter, however, the 0.85-p resonance line alone 
would make a sufficiently large contribution to the 
visual range so that the security requirements for 
communication systems (about 400 yards or less) 
would not be met by a transmitter of sufficiently 
high holo intensity to provide the desired communi¬ 
cation range. Although the 0.89-p resonance line 
makes, on an equal energy basis, an equal contribu¬ 
tion to the range of a communication system, it 
makes a much smaller contribution to the visual 
range because of the rapid decrease in eye response 
between these wavelengths (see Chapter 2). A 
filter having an ideally sharp cutoff between the two 
resonance lines, so that no radiation from the 0.85-p 
line and 100 per cent of the radiation from the 
0.89-p line would be transmitted, would have an ehT 
value of about 40 per cent with reference to a 
cesium-surf ace detector, while the visual range 
would be only about 10 per cent of the value which 
would exist if both resonance lines, but no radiation 
of shorter wavelength, were fully transmitted. Thus 
a sharp cutoff in the desired wavelength region can 
contribute materially to the performance of a sys¬ 
tem utilizing the cesium-vapor lamp. 

The transmission curve of any actual filter rises 
gradually over a fairly wide wavelength band from 
zero to the maximum transmission value. It is pos¬ 
sible to get a much steeper curve with organic dyes 
which may be incorporated in filters of the plastic 
resin type than with the pigments which can be in¬ 
corporated in all-glass filters. Plastic filters devel¬ 
oped by Polaroid Corporation under Contract 
OEMsr-1085 and Ohio State University under Con¬ 
tract OEMsr-987 (see Chapter 2) are found to have 
about equally favorable characteristics for use with 
the cesium-vapor lamp. It is important that the 
transmission cutoff lie between 0.81 p, the upper 
wavelength limit for lines present in the cesium- 
lamp spectrum which contribute much more strongly 
to the visible range than to the operating range, and 
0.85 p, the wavelength of the shorter of the two 
resonance lines in which most of the energy useful 
for infrared communication purposes is transmitted. 

It should also be recalled that, because of the es¬ 
sentially monochromatic quality of the near infrared 






GENERAL DISCUSSION AND RECOMMENDATIONS 


43 


radiation from the cesium lamp, the ehT value of a 
filter for this radiation is essentially the per cent 
transmission of the filter at a wavelength of approx¬ 
imately 0.87 p and is therefore essentially independ¬ 
ent of what near infrared detector is used. In Table 
8 are shown the transmission values of certain filters 
for cesium lamp radiation, together with the visual 
range of a type E transmitter when covered with 
each of these filters. The last two types listed are 
recommended for securing optimum performance of 
a system utilizing the cesium lamp with the “non- 
ideal” filters which are actually available while 
maintaining the highest feasible degree of security 
for military operation. Details concerning the meas¬ 
urement of visual range may be obtained from the 


Table 8. Transmission (ehT) of various filters for 
cesium-lamp radiation and visual range of the type E 
transmitter. 



Transmission 

Visual range 

Filter 

(ehT) 

(yd) 

Corning glass, 3 2 mm, 2566 

0.31 

500 

OSU resin plastic, early sample 
Polaroid sandwich, polyvinyl 

0.59 

700 

alcohol 

0.80 

700 

OSU resin plastic, latest sample 

0.80 

700 


references given above. Other considerations which 
must affect the choice of a filter for military equip¬ 
ment, such as mechanical strength, shock resistance, 
and weathering characteristics, are mentioned in 
Chapter 2. 

14 GENERAL DISCUSSION AND 
RECOMMENDATIONS 

In addition to those discussed above, there exist 
other well-known sources of infrared radiation such 
as the carbon arc and the mercury-vapor arc lamps. 
Both of these types were given preliminary consider¬ 
ation and experimental trials for military applica¬ 
tions in the early stages of the NDRC program. For 
non-image-forming military applications in detec¬ 
tion, ranging, recognition, and code or voice com¬ 
munication to which the near infrared work of 
Section 16.4 was largely confined, they were found 
to be inferior in one or mor& respects to the types of 
sources already described, upon which the program 
of further intensive development was, therefore, 
concentrated. 

The properties of tungsten lamps are well known 


and have been intensively exploited. Incandescent 
lamps will undoubtedly constitute in the future, as 
they have in the past, a most important type of 
source for visible and near infrared radiation, al¬ 
though it seems unlikely that further major ad¬ 
vances will be made in improving their operating 
characteristics and mode of operation. The concen¬ 
trated-arc lamp, whose radiation characteristics are 
in many respects similar to those of the incandescent 
tungsten lamp, is a development of potential im¬ 
portance for certain specialized applications. Addi¬ 
tional improvements in this type of source may be 
possible, and this fact should be borne in mind in 
connection with future military equipment prob¬ 
lems. For some purposes, especially wide-angle, near 
infrared communication systems, the cesium-vapor 
lamp is superior to the concentrated-arc lamp in a 
number of respects, and it is probably capable of 
further improvement also, though primarily in mode 
of construction, shape, and size rather than in fun¬ 
damental mode of operation. Its selective radiation 
characteristics and high efficiency of modulation 
approach the ideal realization of desirable charac¬ 
teristics for a specialized purpose about as closely as 
can be anticipated for any source. The development 
of flash lamps for certain special purposes has been 
highly successful; equally successful developments 
can undoubtedly be made in the future to meet other 
specialized requirements for this type of source. 

At least one other type of source has been shown 
to possess interesting possibilities. Developed in 
France as the source for a near infrared communica¬ 
tion system operating on a radio-frequency carrier 
wave, it consists of a pancake spiral discharge tube 
filled with xenon gas. The possibility of successfully 
applying it in a system of this type has been demon¬ 
strated 30 under Contract OEMsr-1391 by North¬ 
western University (see Chapter 4). It was subse¬ 
quently purchased by the Navy (BuShips) for the 
further investigation and application which it ap¬ 
parently merits. 31 

It is very difficult to outline specific recommenda¬ 
tions for the direction of future development of in¬ 
frared sources. The line of attack will necessarily be 
affected by the military needs of the future, such as 
the size, weight, beam width, electrical character¬ 
istics, operating range, and security requirements, 
the degree of obsolescence of earlier systems through 
general knowledge, and the availability of compo¬ 
nents and their adoption by other countries, etc. 









44 


NEAR INFRARED SOURCES 


A considerable amount of interest has already 
been expressed in systems which would operate at 
a somewhat longer wavelength range than has here¬ 
tofore been employed in the near infrared, namely 
from 1.5 to 3 or 3.5 p. A source such as the incan¬ 
descent tungsten lamp or the concentrated-arc lamp, 
whose radiations consist principally of a thermal 
continuum, emits an appreciable percentage of its 
energy in this region but is considerably less effi¬ 
cient here than for the region near one micron at 
which the peak spectral emission occurs for the oper¬ 
ating temperatures near 3000 K which are ordinar¬ 
ily used to secure the highest feasible holobrightness. 
Decreasing the temperature of the source increases 
the percentage of the total energy radiated at the 
longer wavelengths, but at the expense of a lower 
energy emission per unit area, at each wavelength as 
well as integrated over all wavelengths. However, 
the radiant output in the desired wavelength region 
per watt input slowly increases as the temperature 
of the source is decreased from 3000 K to 2000 K or 
lower. Thus if the area of a source at the lower tem¬ 
perature can be increased in such a way that the 
same spatial distribution of signal flux as at the 
higher temperature is maintained, the operating 
range of a system will remain unchanged and its 
operating efficiency will be somewhat improved. 

In the longer wavelength region the difficulties 


already encountered in electrically modulating such 
a source at the frequencies most useful for communi¬ 
cation purposes will be multiplied still further. Al¬ 
though the Nernst glower radiates selectively near 
2 p, it is of the incandescent type and therefore 
cannot be modulated efficiently by direct electrical 
operation. The development of an electrically modu¬ 
lated lamp as efficient for this region as the cesium- 
vapor lamp is for the 0.9-p region can scarcely be 
anticipated from the well-known spectral emission 
and other pertinent characteristics of the elements. 
It therefore appears likely that, for the immediate 
future, the only really feasible source for extending 
the application of modulated radiation appreciably 
farther into the infrared will emit a continuum from 
which the desired wavelength region will be selected 
by the use of filters developed for this purpose. With 
such a source it appears likely that any modulation 
desired at audio or higher frequencies must be super¬ 
imposed on the radiation by mechanical means after 
it leaves the source, although these means may be 
subject to electrical control methods. The transmis¬ 
sion characteristics of the atmosphere and of the 
envelopes in which the source and the detector are 
constructed will limit the wavelength region to 
which such developments may be extended with any 
degree of success. Other aspects of a possible devel¬ 
opment of this type are discussed in Chapter 4. 




Chapter 2 

NEAR INFRARED TRANSMITTING FILTERS 

By Richard C. Lord a 


21 INTRODUCTION 

he present chapter is devoted to a discussion 
of near infrared transmitting filters used in 
military equipment developed under the auspices of 
Division 16, NDRC. Many of these filters were the 
gutgrowth of investigations sponsored by Section 
16.4 or Section 16.5, but some were the products of 
manufacturers who had no direct relation with 
NDRC in this connection. A minimum of back¬ 
ground information on the general subject of infra¬ 
red filters is included. Such information is widely 
available in scientific texts and journals and has 
been assembled in a comprehensive report. 1 

2 2 DEFINITION OF TERMS 

The following terms are helpful in discussion of 
the properties and practical applications of infrared 
filters (see Appendix, Tables I and II). 

Relative spectral responsivity, denoted r^, is the 
responsivity at any wavelength X of a detector 
expressed in terms of an arbitrary scale in which the 
peak value of the responsivity is taken as unity. All 
other values are thus less than unity. This conven¬ 
tion of setting the maximum value of a relative 
function equal to unity will be observed for the fol¬ 
lowing quantities as well as for r x . 

Relative luminosity, Rx, is the relative spectral 
responsivity of the human eye. 

Relative spectral radiant flux, <f> x , is the flux in 
watts per micron at a particular wavelength X of a 
radiant source for which the maximum value is 1 
watt per micron. 

The fractional transmission, T x , of radiation of 
wavelength X by a filter is defined as the ratio of the 
total radiant flux of wavelength X emergent from one 
face of the filter to the total radiant flux of the same 
wavelength incident on the other face of the filter. 
The path of the radiation in the filter may or may 
not be rectilinear. If the above definition is further 
restricted to radiant flux transmitted rectilinearly 
by the filter, the transmission so defined is called the 
fractional specular transmission. Clearly, the frac- 
a Massachusetts Institute of Technology. 


tional specular transmission is smaller than the frac¬ 
tional transmission, and also the fractional trans¬ 
mission is no greater than unity. 

With the help of the above concepts, the two 
quantities of fundamental importance in determin¬ 
ing the performance of infrared filters in military 
devices may be defined. These two quantities are 
the effective hololuminous transmission, ehT, and 
effective visual trans?7iission, evT. The former meas¬ 
ures the efficacy of the filter in passing useful infra¬ 
red radiation, and the latter the efficacy of the filter 
in blocking out unwanted visible radiation. In math¬ 
ematical terms, 

f <t>>T>r,A 

ehT = ^-, (1) 

J 0 

[ 4>iTxRxdX 

and evT = -°-. (2) 

/•X 

/ 4>\R\dX 

J o 

The indicated integrations extend in principle from 
X — 0 to X — °o. In practice, the X range need not be 
greater than from X — 0.4 p to X = 1.4 p, for the 
usual military sources and receivers, and frequently 
it is much smaller still. 

It is clear from equations (1) and (2) that ehT 
and evT, that is to say, the performance character¬ 
istics of an infrared filter, do not depend on proper¬ 
ties of the filter (T\) alone, but also on the infrared 
source-receiver combination with which the filter is 
used. The source and receiver must be stated or im¬ 
plied whenever ehT and evT values are given for an 
infrared filter. The convention has been adopted (see 
Appendix) of denoting by ehT° those ehT values 
obtained with a standard tungsten lamp operated at 
a color temperature of 2848 K. Actual or approxi¬ 
mate relative spectral radiant flux functions (</>*) 
are given for several infrared sources in Chapter 1, 
Figure 14. b Similarly, in Chapter 3 the relative spec¬ 
tral responsivity of the various infrared-sensitive 
photocells is indicated. 

b See also Summary Technical Report, Division 16, Vol¬ 
ume 4, Chapter 5, Figures 7 and 29. 


liESEBSSSE© 


45 








46 


NEAR INFRARED TRANSMITTING FILTERS 


2-2,1 Determination of ehT 

Irrespective of the nature of the military equip¬ 
ment concerned, if near infrared radiation is the 
basis of operation, the performance of the equip¬ 
ment is optimum if ehT is highest and evT lowest. 
Determination of these quantities is therefore pre¬ 
requisite to a decision as to what filter is the best 
to use in a particular device. 

Devices for the measurement of ehT have been 
described in NDRC and other reports. 2 - 3 ’ 4 These 
devices are essentially alike, consisting of a stand¬ 
ard source of radiation, an optical system for col¬ 
lecting the radiation from the source and focusing 
it on the receiver, and the standard receiver itself. 
In addition, there is provision for insertion of a 
filter in the optical path at one place or another, 
depending on whether ehT or ehT (specular) is to 
be measured. The electrical output of the photocell 
receiver is amplified and measured in conventional 
fashion. Measurement of ehT is made by reading the 
output of the receiver when the source is emitting 
radiation with no infrared filter in the radiation 
path, and then reading the receiver output with the 
infrared filter placed in the optical path. The ratio 
of the latter reading to the former is the ehT of the 
filter for the particular source-receiver combination 
used, provided the readings obtained are linearly 
proportional to the radiation incident on the re¬ 
ceiver. Clearly, ehT measured in this fashion must 
be numerically less than unity. Whether the value 
measured corresponds more closely to ehT or to 


ehT (specular) depends on the placement of the fil¬ 
ter in the optical path. If the placement of the filter 
is made at a point where the radiation transmitted 
nonrectilinearly by the filter escapes from the opti¬ 
cal path, ehT (specular) is determined, otherwise 
ehT. 

It is possible to compute ehT for an infrared filter 
with respect to any source-receiver combination, 
when the numerical values of fa, 7\ and are avail¬ 
able at all pertinent wavelengths, by numerical in¬ 
tegration according to equation (1). This procedure 
is somewhat laborious, although the results are re¬ 
liable when the fa, and r* values are fairly accu¬ 
rate. The method of finding 7\, however, is just as 
difficult for one wavelength as is ehT for the entire 
range of wavelengths, so that the computation of 
ehT is of no advantage over direct measurement ex¬ 
cept in special situations. One such circumstance is 
that in which, for some reason, fa or T\ or r x is 
known, but the actual source or receiver or filter is 
not available for use in the determination of ehT. 

Table 1 shows ehT values for various filters. All 
these values are for cesium-surface photocells, the 
source being incandescent tungsten at 2800 to 
3000 K. 

2-22 Determination of evT 

The measurement of evT is attended by more 
difficulty than that of ehT. The primary reason for 
this is the fact that one desirable quality of an 
infrared filter is a minimum value of evT. Since this 


Table 1 . Values of ehT and evT for various near infrared filters. 


Filter type 

Base material 

ehT range 

evT range 

Density 

Corning 2540 

Glass (2.6 mm) 

0.13 

io- 7 

Medium high 

Corning 2568 

Glass (7.5 mm) 

0.20 

io- 5 

Low 

Polaroid XR3X 

Cellophane, nylon 

0.25-0.4 

io- a -IO' 5 

Low 

Polaroid XR7X 

Cellophane, nylon 

0.05-0.15 

IO" 9 -2 X 10" 8 

High 

Polaroid XRN1X 

Polyvinyl alcohol 

0.25-0.4 

io- 6 -io- 5 

Low 

Polaroid XRN2X 

Polyvinyl alcohol 

0.07-0.17 

o 

l? 

© 

OB 

High 

Polaroid XRN5X 

Polyvinyl alcohol 

0.3 -0.4 

5 X IO" 7 -5 X IO" 6 

Low 

Ohio State Type I * 

Melmac-Rezyl resin 

0.5 -0.7 

IO’ 5 -5 X IO' 4 

Low 

Ohio State Type II * 

Melmac-Rezyl resin 

0.4 -0.6 

IO" 6 -10" 4 

Low 

Ohio State Type III* 

Melmac-Rezyl resin 

0.2 -0.4 

10~ 8 -IO' 6 

Medium high 

Ohio State Type IV * 

Melmac-Rezyl resin 

0.05-0.2 

10 _1 °-5 X IO' 8 

High 


* The type numbers I, II, III, IV have not been used by the Ohio State group but are used here for the sake of brevity. They refer serially to 
the four dyes for which data are summarized on page 19a of reference 12. 


•re stricted ' 








MILITARY REQUIREMENTS FOR INFRARED FILTERS 


47 


minimum value, for practical purposes, is seldom 
larger than 10' 6 , whereas ehT is seldom as small 
as 10~ 2 , it is readily understandable that evT is 
harder to measure. Moreover, the calculation of 
evT is attended by equal difficulty because rea¬ 
sonably accurate values of T*, when 7\ is as small as 
10" 4 , and approximate values of 7\, when 7\ is as 
small as 10" 7 , are required. 

There are two essentially different methods of 
measuring evT, a direct and an indirect method. 
The former makes use of a standard infrared filter 
whose evT is already known and involves a visual 
comparison, under controlled conditions, of the un¬ 
known with the standard filter. Various optical ar¬ 
rangements for this comparison, based on traditional 
techniques in visual photometry, have been used in 
Army, Navy, and NDRC laboratories in this coun¬ 
try and by the Admiralty Research Laboratory in 
England. For example, the U. S. Naval Research 
Laboratory has modified the commercially available 
Macbeth illuminometer to enable evT’s as small as 
10 -8 to be measured with good accuracy. 38 Another 
arrangement which is similar to the above, but 
which enables simultaneous comparison of the un¬ 
known filter with two standards of any arbitrary 
difference in evT, is described in a report, 128 and a 
British procedure for photometric measurement of 
evT is reported. 5 

The indirect method for finding evT consists of a 
measurement of the normal visual range [NVR], 
which is the visual range limit of a particular source 
viewed by the dark-adapted eye in total darkness. 
The NVR for a filter of unknown evT can be con¬ 
verted to evT with the help of the relationship 

evT = Constant — . (3) 

cp 

The value of the constant in the equation is best 
evaluated empirically for a particular observer or 
set of observers by measurement of NVR for a filter 
of known and similar evT used in conjunction with 
a source of known candlepower, cp. 

Determination of evT in this fashion is beset with 
many sources of error. Some of these are described 
in a report 6 summarizing extensive studies on the 
visual range of various infrared sources carried out 
at Brown University (under Section 16.1). The 
reader is referred to this report for details on various 
methods of measurement of NVR itself, and the 
advantages and disadvantages of each. Despite the 


difficulty of getting consistent, reproducible, and 
accurate NVR values, use of this quantity is per¬ 
haps the most realistic way of evaluating evT for 
filters to be used for military purposes, especially 
for filters with very low evT ranges (evT = 10" 9 ). 

23 MILITARY REQUIREMENTS FOR 
INFRARED FILTERS 

As the war progressed, it became apparent that 
the military requirements for infrared filters were 
of two different, and to some extent contradictory, 
sorts. Of great importance, of course, was the evT- 
ehT requirement, which may be termed the spectral 
requirement. In addition, however, physical rugged¬ 
ness of filters was perhaps even more essential 
because of the severe treatment of all infrared 
equipment during field use. Under the heading of 
physical ruggedness one should include resistance 
to mechanical and thermal punishment of all kinds, 
as well as resistance to weathering, particularly to 
exposure to sunlight and salt spray. The physical 
endurance required of an infrared filter in a piece 
of military equipment varies greatly, of course, the 
demands being less on filters incorporated in intri¬ 
cate equipment such as an infrared phototelephone 
and more severe in a simple device such as a ship¬ 
board beacon. In general, however, it may be said 
that ruggedness is a requirement second to none 
regardless of the application involved. Moreover, it 
is agreed by those concerned with filter use for mili¬ 
tary purposes that there is still much room for im¬ 
provement in the physical characteristics of filters. 48 

Most of the NDRC-sponsored work on infrared 
filters was devoted to the development of filters of 
improved spectral characteristics. The spectral re¬ 
quirements for infrared filters for military use fall 
rather distinctly into two separate kinds, that for 
very low evT values, approaching 10" 10 , and that 
for which higher evT values, 10“ 8 or even larger, are 
tolerable. We shall refer to these two types of filters 
as “high density” and “low density” respectively. 

231 Requirements for High-Density 
Filters 

From the fact that devices using near infrared 
radiation have an energy threshold below which 
they will not function, it is clear that such devices 
will function at longer ranges if more infrared radia- 








48 


NEAR INFRARED TRANSMITTING FILTERS 


tion is available. When this radiation is furnished 
by a filtered source, the higher the ehT of the filter, 
the larger is the amount of infrared radiation ob¬ 
tainable from that particular source (Chapter 1). 
Consequently, all filters should have as high an ehT 
as is consistent with other requirements, but these 
other requirements may demand a very low evT. 
Since high evT and high ehT in general go together, 
high ehT may have to be sacrificed if low evT is 
essential. 

The military circumstances under which high- 
density filters are required may be described roughly 
as follows: If the military usefulness of an infrared 
device depends on the probability of visual detec¬ 
tion of the device being as low as, say, 1 in 1,000, by 
enemy personnel at ranges of the order of 10 yards, 
a high-density filter is necessary. Of course, range 
of detection can be reduced by reducing the radiant 
flux of the source, but operating range is corre¬ 
spondingly reduced. 

From equations (1) and (2) it can be seen that 
ehT and evT depend not alone on 7\, but on <j> X} r x 
(for ehT) and Rx (for evT) as well. For incandes¬ 
cent filament sources, <£ x is pretty well the same for 
one source as for another, so that an examination of 
r x and Rx will be more indicative. The r x curves for 
cesium-surface photoelectric cells and for thallous 
sulfide photoconductive cells are shown in Figure 1, 
and R x , plotted on a logarithmic scale, in Figure 2. c 

Comparison of Figures 1 and 2 shows that the 
human eye has residual sensitivity in the spectral 
region around 0.8 to 0.9 \i, where a considerable 
part of the useful sensitivity of the thallous sulfide 
and cesium photocells also lies. Hence a filter de¬ 
signed to absorb radiation in this region to reduce 
evT also reduces ehT. The question therefore arises 
as to the best compromise to make. 

It can be shown la that the best high-density 
filters are not necessarily those with the steepest 7\ 

c The R^ curve is for photopic rather than scotopic vision. 
However, the slope of the photopic curve and that of the sco¬ 
topic curve are very nearly equal throughout the extreme red 
from about 0.7 to 1.0 p. Thus. evT’s calculated from the two 
different curves for R x differ only by a constant factor so long 
as T x for the filter under consideration is essentially zero at 
wavelengths less than 0.7 p (for military purposes. R x may 
be considered as zero at wavelengths longer than 1.0 p). 
This constant factor is swallowed up in the empirical constant 
in equation (3) if evT’s are determined by NVR measure¬ 
ment. If evT’s are determined photometrically, the condi¬ 
tions of measurement decide whether they correspond to 
photopic or scotopic values. In the present volume, all 
evT values are based on the photopic curve. 


curves in the region between 0.8 and 0.9 u. Rather, 
the important point is to achieve the necessary low 
evT by making sure that the steep part of the 7\ 
curve lies at sufficiently long wavelengths. This 



0 i------ 

0.5 0.6 0.7 0.8 0.9 1.0 l.l 1.2 1.3 1.4 


WAVELENGTH IN MICRONS 

Figure 1 . Relative responsivity for cesium and thal¬ 
lous sulfide photocells. 

brings about some reduction in ehT, to be sure, but 
the price must be paid if high-density filters are 
required. Examples of ehT and evT for typical high- 
density filters for use with the Sniperscope d are 
listed in Table 1. 






























► 







i 







* 





LINE IN0I 
•OLATION 

ICATES — 





BROKEN 

EXTRAF 





\ 

V 







\ 

V 

V 




i 



/ 

/ 

/ 



1 




X 

_ 


0.5 0.6 0.7 0.8 0.9 IX) l.l l.t 


WAVELENGTH IN MICRONS 

Figure 2. Logarithm of relative luminosity as a 
function of wavelength for cesium and thallous sulfide 
photocells. 

The only cure for the deficiency in ehT of high- 
density filters is to make use of near infrared de¬ 
tectors whose sensitivity curves have their maximum 
well beyond 1 u (Chapter 3). With such detectors, 
it would be possible to make filters of indefinitely 

d See Sum man* Technical Report. Division 16. Volume 4. 
Chapter 2. 




















































STATUS OF INFRARED FILTERS PRIOR TO 1941 


49 


low evT (< 10“ 12 ) while maintaining relatively 
high ehT (say 0.5). 

Requirements for Low-Density 
Filters 

There are many military uses for infrared devices 
where much longer visual ranges for the infrared 
sources are tolerable, as, for example, with infrared 
communications systems aboard naval vessels 
(Chapter 4). Here the visual range may rise to 
several hundred yards or more without impairing 
the security of the device in operational use. Ac¬ 
cordingly, an increase in ehT may be had to the 
extent allowed by whatever increase in evT is per¬ 
missible. The percentage gain in ehT which results 
from a given percentage increase in evT is always 
many times smaller than the latter but may result 
in considerably increased operating range. 

The precise requirements for low-density filters 
depend on the nature of the source-receiver com¬ 
bination. If the source is incandescent tungsten at 
about 3000 K and the receiver a thallous sulfide or 
cesium-surface photocell, a filter with a steep 
curve e located approximately at the right wave¬ 
lengths for the evT requirements will give satisfac¬ 
tory ehT values. If the source is a special one, such 
as the cesium-vapor lamp, in which the spectral 
distribution of the radiation has two sharp maxima 
with a deep minimum between (Chapter 1, Table 7), 
the location of the steep portion of the 7\ curve may 
be quite critical for the ehT/evT ratio. 

2,4 STATUS OF INFRARED FILTERS 
PRIOR TO 1941 

There were several commercially available infra¬ 
red filters in the prewar years, notably the glass 
filters of Jena and Corning (for example, Jena RG-7 
and Corning No. 254) and the Wratten-dyed gelatin 
filters (Nos. 87 and 88a). These were designed for 
scientific and other nonmilitary uses and in general 
had shortcomings which made them unsuitable for 
military application. The glass filters could be made 
in sufficiently thick slabs to serve as high-density 

e By “steep” curve is meant a T k curve such that 
(1 /Ty)(dT x /dX) is of the order of 50 reciprocal microns 
when T^ is less than 0.5. Plastic filters approach or better 
this value, while glass filters usually have a value of around 
10 to 20. It may be noted that (l/R^)(dR^/d\) has a value 
of —70 reciprocal microns at 0.8 p. 


filters, but because of the comparatively gentle slope 
of their T\ curves, the corresponding ehT values 
were undesirably small. In addition, the glass was 
thermally fragile. The dyed gelatin filters, on the 
other hand, were of too high evT even for low- 
density military uses, although the ehT was quite 
satisfactory. The unmounted gelatin film, of course, 
would be hopelessly delicate for military use, apart 
from its low heat resistance. 

Thus it was apparent that research on near infra¬ 
red filters should follow two main paths: (1) the 
development of physically rugged filters, and (2) 
the development of improved spectral characteris¬ 
tics, especially lower evT values and improved 
ehT/evT ratios. 

Since glass was the most promising filter material 
from a mechanical standpoint, a program was under¬ 
taken by Corning Glass Works in this country and 
independently by Chance Brothers in England to 
increase the heat resistance of the glass used in 
infrared filters. In addition, study was made of the 
possibility of improving the 7\ curve for glass filters 

1 CORNING NO, 2540-0.34 MM 

2 CORNING NO. 2540“ 1.96 MM 

3 CORNING NO. 2540- 4 MM APPROX 

4 JENA RG- 7- 1 MM 



Figure 3. Transmission curves for several glass filters. 

by use of new pigments or other means. The work 
done by Corning was on that firm’s own initiative, 
although NDRC and Army and Navy laboratories 
were kept informed of progress and supplied with 
samples. The results of the work were disappoint¬ 
ing, in so far as the 7\ curves are concerned. While 
glass infrared filters of vastly improved thermal 
properties were developed, the Corning 2560 series, 
the transmission curves of these glasses differed 
very little, when allowance is made for sample thick¬ 
ness, from that of the old No. 254 glass. A set of 
glass infrared filter transmission curves is shown in 
Figure 3. 






















50 


NEAR INFRARED TRANSMITTING FILTERS 


25 INFRARED FILTERS DEVELOPED 
BY NDRC (1942-45) 

The infrared filter developments sponsored by 
Division 16 of NDRC had as their object the inves¬ 
tigation of the use of organic dyes in various media. 
Dyes have much steeper transmission curves than 
those of the pigments used in glass infrared filters, 
and, in addition, the location of the transmission 
curve on the wavelength scale can be determined 
almost at will by proper selection of the dye. 

The organic dye has to be incorporated in some 
base, of course, and since the physical properties 
of the filter are no better than those of the base, the 
development of a good base is as important as 
finding a good dye. In fact, experience has shown 
it to be more important. The number of optically 
satisfactory materials in which organic dyes can be 
put is considerable but not infinite, and is limited 
essentially to plastics, whether natural or synthetic. 
Of the two NDRC-sponsored projects on infrared 
filters, one was concerned essentially with the study 
of various plastic materials which can be dyed in 
the form of thin sheets, while the other investigated 
plastics in which the dye could be incorporated in 
the monomeric form prior to the polymerization 
process. The results of these two projects will be 
summarized briefly. 

2,5,1 Polaroid Filter Investigation 13 

Because of its experience with the commercial 
production of optical filters of various kinds, the 
Polaroid Corporation was asked to conduct a pro¬ 
gram for the development of improved dyed plastic 
filters. Prior to the beginning of this program, 
Polaroid had on its own initiative developed and 
put into pilot production its XRX type of filter. 
This type consisted of cellophane sheet vat-dyed 
with appropriate dyes to give either low-density or 
high-density filters. The former were denoted XR3X 
and the latter XR5X and XR7X. The proper den¬ 
sity within each type could be obtained by adjust¬ 
ment of the dyeing time, longer time in the dyeing 
vats giving higher densities. Most of these filters 
were made with two dyes, a red and a blue, in order 
to provide the necessary high density throughout 
the wavelength region from 0.4 to 0.8 p. 

The transmission curves of several cellophane 
XRX filters are shown in Figure 4, and ehT and 


evT values are given in Table 1. It can be seen that 
both for high-density and low-density types the 
ehT and evT are quite satisfactory. 

The fabrication of a finished filter from the dyed 
cellophane sheet consisted of bonding the sheet to a 
glass support of the desired size and shape. The 
sheet could be applied without difficulty to glass 
flats of various shapes, to cylinders, and even to 
hemispheres. Since the bonding material was usually 
polyvinyl alcohol, which is pervious to water, the 
filter was coated with a waterproof varnish. 

1 POLAROID XR3X (CELLOPHANE AND NYLON) 

2 POLAROID XR7X (CELLOPHANE AND NYLON) 

3 POLAROID XRN1X (POLYVINYL ALCOHOL) 

4 POLAROID XRN2X (POLYVINYL ALCOHOL) 



O 1 —^^^--- 

0.7 0.8 0.9 1.0 l.l 1.2 


WAVELENGTH IN MICRONS 

Figure 4. Transmission curves for several XRX 

filters. 

Heat Resistance 

The chief shortcoming of the glass-supported cel¬ 
lophane filter is its poor heat resistance. This diffi¬ 
culty is associated with the cellophane base itself 
and not with the vat dyes, which are stable at much 
higher temperatures than the cellophane. In fact 
while the dyes themselves will remain stable at tem¬ 
peratures of 200 C or higher for many hours, cello¬ 
phane becomes brittle and cracks after relatively 
short exposure (several hours) to temperatures 
around 120 to 130 C. In addition, cellophane filters 
in field operation and in severe weathering tests 
showed tendencies toward formation of pinholes 
and cracks. 

Accordingly the Polaroid contract had as its first 
objective the finding, if possible, of an optically 



















INFRARED FILTERS DEVELOPED BY NDRC (1942-45) 


51 


clear plastic sheet which would accept dye as satis¬ 
factorily as cellophane but would be much more 
heat- and weather-resistant. 

Nylon Filters 

After investigating a number of plastic bases, 
Polaroid found that nylon film, duPont type 6A, 
which has much better heat stability than cello¬ 
phane, could be dyed satisfactorily with the same 
dyes used in the XRX filters. Curves of for typi¬ 
cal dyed nylon sheet filters are shown in Figure 4, 
and approximate ehT and evT values are given in 
Table 1. 

The dyed nylon sheet was bonded to glass sup¬ 
ports in appropriate shapes for various military 
applications. The bonding technique was unusual 
because of the special character of nylon. A phenol- 
formaldehyde cement, Chrysler Cycle-Weld 55-6, 
was the bonding agent, and the bonding process was 
carried out under pressure (4 to 5 atmospheres) and 
at elevated temperatures (about 150 C). 

Extensive testing of the nylon XRX type of filter 
indicates that from an optical point of view its 
qualities are quite satisfactory. There is no doubt 
that nylon filters are much more stable to heat, and 
particularly to alternate heat and cold, than are cel¬ 
lophane filters. The weathering properties of nylon 
filters are also much superior to those of cellophane. 
However, in both respects nylon filters still leave 
much to be desired. In particular, the heat resistance 
of the nylon base is still inferior to that of the 
filter dyes. Accordingly, vat-dyed filters of still 
better heat stability are possible, even with present 
dyes. 

Evaporated Filters 

The fact that the vat dyes are more heat-stable 
than their plastic bases led to an interesting filter 
development which may have possibilities, even 
though, for practical reasons, these were not ex¬ 
ploited during the lifetime of Contract OEMsr-1085. 
Polaroid found that the dyes could be evaporated 
in vacuo and deposited in thin films on a supporting 
base in the manner employed in the evaporation 
of metals. Numerous filters of excellent spectral 
properties and highly improved heat stability were 
made in this fashion. The project was abandoned 
because of difficulties with quantity production of 
evaporated filters. It is conceivable, however, that 
these difficulties could be overcome and that evap¬ 


orated filters might be a practicable answer to the 
quest for heat-stable dye filters. 

PVA Filters 

While work was in progress on filters of improved 
heat and weather stability, a special need arose for 
low-density filters of favorable ehT to evT ratio. 
These filters were to be used with low temperature 
sources, the cesium-vapor lamp and the type D-2 
beacon (see Chapter 1), in such a way that neither 
the heat stability nor weathering properties were of 
paramount importance. Accordingly, it was possible 
to concentrate attention on the spectral properties 
of the filter. 

The success of the British in making excellent 
low-density filters with a polyvinyl alcohol [PVA] 
base suggested this plastic for trial. It was soon 
found that cast PVA sheet, in which the dye had 
been incorporated before casting, gave filters of the 
best optical quality yet obtained. The problem of 
producing a satisfactory filter thus was reduced to 
that of finding the best PVA-soluble dye for the pur¬ 
pose and the best way of mounting the PVA sheet. 

Several hundred dyes were studied in the devel¬ 
opment of the PVA series (denoted XRNX), the 
best of which are described in a report. 138 curves 
as well as ehT and evT values for several typical 
XRNX filters are given in Figure 4 and Table 1. 
Field trials with equipment using the cesium-vapor 
lamp as a source (e.g., type E equipment described 
in Section 4.4.2) showed that properly selected PVA 
filters gave very high efficiency, as judged by the 
ratio of operating range to visual range. 

Because PVA sheet is vulnerable to moisture, it 
is best mounted on glass in a sandwich type of 
lamination. The PVA lends itself well to standard 
laminating procedures, but it is necessary to pay 
special attention to the edge-sealing of the sand¬ 
wich. Another important point is the kind of glass 
used in the sandwich. Many commercial glasses 
with a greenish tinge show pronounced absorption 
at around 0.9 p. Such glass can reduce the ehT of 
the sandwich by 10 per cent or more and should not 
be used. It is easy to obtain satisfactory glass, pro¬ 
vided one remembers to check the of the glass in 
the 0.8- to 1.2-p region. 

Other Polaroid Research 

The remainder of the Polaroid program was de¬ 
voted to the development of filters suitable for 



52 


NEAR INFRARED TRANSMITTING FILTERS 


sources and receivers operating at wavelengths 
longer than 1.4 \i. This work was begun toward the 
end of the program and will be discussed briefly in 
Section 2.7. 

2 5 2 Ohio State Research Foundation 
Filter Investigation 12 

The second of the two NDRC-sponsored infrared 
filter projects was initiated originally for a some¬ 
what different purpose from that of the Polaroid 
project. In mid-1943, it appeared that enemy use of 
the near infrared, as defined by the cesium-surfaced 
photocell, made imperative an extension of our 
infrared devices, both image-forming and non¬ 
image-forming, into the wavelength region beyond 
1.4 p. One promising approach lay through the 
development of infrared-sensitive phosphors, and a 
large program was initiated with this objective/ 

Filters for Radiation Beyond 1.4 Microns 

In order to utilize radiation in the region beyond 
1.4 p, without at the same time violating security 
from an enemy equipped with cesium-surfaced de¬ 
tecting equipment, infrared filters are needed which 
will transmit freely radiation at 1.4 p and longer, 
and strongly absorb shorter wavelengths. The group 
in the Laboratory of Chemical Spectroscopy at Ohio 
State University was asked to develop such filters. 
This group was already participating in the phos¬ 
phor program as a testing laboratory, and was 
familiar with the scientific and military background 
of the proposed filter investigation. The Ohio State 
University Contract OEMsr-987 was set up and 
administered by Section 16.5, but its filter develop¬ 
ments are reported here rather than in Volume 4 for 
the sake of a unified discussion of filters. In actual 
practice, the Ohio State filters were developed for, 
and used in conjunction with, non-image-forming 
devices of Section 16.4 as well as for the image-form¬ 
ing devices described in Volume 4. 

The first approach to the problem of making a 
filter with vanishingly small transmission at 1 p was 
to examine the absorption spectra of various sub¬ 
stances, both organic and inorganic, in this region. 
It appeared that copper salts in solution have satis¬ 
factory absorption characteristics. After much ex¬ 
perimentation, a filter consisting of 30 per cent 

f See Summary Technical Report of Division 16, Vol¬ 
ume 4. 


copper oleate dissolved in oleic acid was developed. 
At the elevated temperatures of operational use, the 
filter medium was liquid, so that the filter consisted 
of a liquid-tight cell with sylphon bellows as an 
integral part of the cell. The bellows expanded with 
temperature-dilation of the liquid. The curve for 
a typical copper oleate filter is shown in Figure 5. 

1 OHIO STATE MELMAC TYPE I 

2 OHIO STATE MELMAC TYPE 3H 

3 OHIO STATE MELMAC TYPE TSL 

4 OHIO STATE MELMAC TYPE 3E 

5 OHIO STATE LIQUID FILTER 



0.5 0.6 0.7 0.8 0.9 1.0 l.l 1.2 1.3 


WAVELENGTH IN MICRONS 

Figure 5. Infrared transmission curves for Melmac 

and liquid filters. 

Filters for Shorter Wavelengths 

The objectives of the Ohio State project changed 
as a result of two occurrences. The first was the fail¬ 
ure of the phosphor program to produce a phosphor 
with adequate sensitivity at wavelengths beyond 
1.2 p, so that a need for longer wavelength filters did 
not materialize. The second was the increased de¬ 
mand for improved infrared filters at the shorter 
wavelengths. In the early part of 1944, therefore, the 
project began the study of plastic media suitable 
for filters and of infrared-transparent organic dyes 
which could be incorporated in these media. 

The Melmac Plastic Filter. A considerable pro¬ 
gram of study of plastics was undertaken. As has 
been indicated in earlier discussion, the primary 
consideration in the choice of a plastic base is physi¬ 
cal ruggedness, heat, and weather resistance in 
particular. The Ohio State group, after testing nu¬ 
merous plastics, reached the conclusion that a com¬ 
mercial melamine resin known as Melmac 599-8, 
when combined with the proper proportions (ap¬ 
proximately 1 to 1) of an alkyd resin Rezyl 330-5, 
showed the best overall properties. The Melmac 
plastic has good heat resistance at 150 C, excellent 
weather stability and satisfactory mechanical prop- 

























FUTURE DEVELOPMENTS IN MILITARY INFRARED FILTERS 


53 


erties. It takes appropriate dyes and the optical 
properties of the dyed plastic are excellent. 

The best feature of the Melmac plastic is the ex¬ 
cellent bond it forms to a clean glass surface. This 
bond serves as the basis for fabrication of filters 
from the resin. In the fabrication of flat filters, such 
as the coated flashlight disks furnished by the Ohio 
State group to the Signal Corps, 11 the resin with the 
incorporated dye is poured in proper amount upon 
the carefully leveled surface of the disk to be coated. 
The film is allowed to dry in air for several hours, 
and is then polymerized by baking at a temperature 
of 100 C for a like period or longer. The resultant 
resin-to-glass bond is so strong that the glass or 
plastic will fracture before the bond. 

An additional useful feature of the Melmac resin 
filter material is that dilution of the dyed resin with 
20 to 30 per cent butyl Cellosolve makes it suitable 
for spraying in conventional fashion. In this way 
complicated nonplanar surfaces can be coated. It is 
difficult, of course, to secure uniform filter thickness 
in a sprayed coat, but the same difficulty is encoun¬ 
tered whenever complicated filters are fabricated 
from glass or molded plastic. 

Filter Research and Production 

The Ohio State group carried out a survey of suit¬ 
able organic dyes similar to that made by the 
Polaroid Corporation and described above. The two 
surveys were not duplication of effort, since the 
Polaroid search was for water-soluble and the Ohio 
State search for spirit-soluble dyes. A list of dyes 
found to be satisfactory is given in a report, 12b and 
curves for filters made with some of these are 
given in Figure 5. Values of ehT and evT are given 
in Table 1. 

In addition to its research program, the Ohio 
State group did a considerable amount of actual 
production of its filters, and also aided the Armed 
Services in the establishment of production at other 
locations. The work of the group will be continued 
under the sponsorship of the U. S. Army Corps of 
Engineers. 

2 6 USE OF FILTERS IN 

MILITARY DEVICES 

It is not necessary to discuss here the specific 
applications of the filters just described to military 
instruments, inasmuch as this subject is considered 


in detail in other parts of this volume and in Vol¬ 
ume 4. The use of filters with various sources, re¬ 
ceivers, signaling and communications systems, as 
well as with beacons, markers, and other infrared 
devices is adequately described in contractors’ re¬ 
ports concerning the development of these devices, 
and also in other parts of the Summary Technical 
Report. 

27 FUTURE DEVELOPMENTS IN MILITARY 
INFRARED FILTERS 

2 71 Near Infrared Filters 

Ample room remains for the improvement of near 
infrared filters. The filters of best mechanical prop¬ 
erties (glass) have poorest spectral qualities, and 
there appears to be no intrinsic reason why glass 
pigments cannot be found which give much improved 
transmission curves. Certainly one may expect con¬ 
siderable improvement in plastic filters to result 
from the discovery of new plastics, or the exploita¬ 
tion of old plastics not hitherto used. There is rea¬ 
son to doubt, however, whether any considerable 
research effort directed toward this improvement 
would be justified. The future of military infrared 
appears to lie in wavelength regions well beyond 
1 p, where the eye has no usable sensitivity, so that 
no compromise between ehT and evT is necessary. 

Intermediate Infrared Filters 

The possibility of discovering infrared phosphors 
sensitive to wavelengths above 1.4 p prompted the 
first investigation of longer wavelength filters. Later, 
when we learned that the Germans had developed 
the lead sulfide detector, whose sensitivity prevails 
out beyond 3 p, the need for intermediate infrared 
filters became apparent. Toward the end of the war, 
the Polaroid group was asked to consider the devel¬ 
opment of infrared filters transmitting less than 1 
per cent at 1.4 p and as much as possible at longer 
wavelengths. Four types of filters were investigated, 
no one of which was completely studied. These were 
metallic sulfide layer filters; furan plastic filters; 
green glass XRNX filters; and sulfur-dyed PVA 
filters. All of these show considerable promise and 
should be studied further for ehT values with re¬ 
spect to lead sulfide cells. In addition other possi¬ 
bilities such as interference layer filters should be 
explored. 




54 


NEAR INFRARED TRANSMITTING FILTERS 


If image-forming devices using lead sulfide or 
other materials sensitive in the intermediate infra¬ 
red are developed, it appears likely that they will 
supersede the shorter wavelength devices. In such 
an event, a vigorous research program on inter¬ 
mediate infrared filters is a necessity. 

2 7,3 Far Infrared Filters 

It is beyond the scope of this chapter to discuss 
infrared filters for the 10-p region, on which some 
work was done during the war. Since these filters 
were not primarily security devices but were for the 
improvement of the sensitivity of heat detectors, 
their efficiency did not have to be great. If image¬ 
forming devices for the 10-p region are developed 
or if strong, modulable sources of 10-p radiation can 
be made, the status of the far infrared region will 
resemble that of the intermediate infrared. At the 
present time this prospect seems rather distant, and 
the need for research on far infrared filters is not 
pressing. 

28 SUMMARY 

The near infrared filters available commercially 
at the end of 1941 were not suitable for use in most 
of the military infrared devices then and later 
developed by NDRC contractors. Accordingly, an 
NDRC-sponsored program of improvement in filters 


was initiated. The objectives of the program were 
to obtain filters of better spectral quality and of 
greatly increased ruggedness. Two projects were set 
up, one at the Polaroid Corporation, sponsored by 
Section 16.4, NDRC, and one at Ohio State Uni¬ 
versity, sponsored by Section 16.5. 

The Polaroid project developed vat-dyed plastic 
sheet filters using cellophane, nylon, and polyvinyl 
alcohol sheet. The dyed sheet was supported by 
bonding it to glass. These filters had excellent spec¬ 
tral quality and a wide range of densities, but their 
low heat stability did not permit their use with 
high-powered sources. 

The Ohio State project developed a dyed lacquer 
filter with a melamine resin base. The dyes which 
could be incorporated in this base gave the resultant 
filters excellent spectral quality, and the physical 
characteristics of the resin were also quite good. 
However, it, too, was unstable at high temperatures 
and could not be used with high-powered sources. 

Much room for improvement in the heat resist¬ 
ance of near infrared filters still exists. Whether the 
necessary research effort is worth expending, how¬ 
ever, is dependent upon future developments in the 
fields of intermediate (1.5 to 5 p) and far (5 to 15 p) 
infrared. In particular, if intermediate infrared 
sources and detecting devices are developed with 
performances comparable to those of the devices now 
available in the near infrared, extensive work in the 
field of intermediate infrared filters will be called 
for. 



Chapter 3 

NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 

By John R. Platt a 


31 INTRODUCTION 

Object and Scope of Chapter 

T his chapter is concerned with developments, 
primarily those sponsored by NDRC, in the 
field of near infrared [NIR] detecting devices. 
These are devices which give a signal in response to 
the total amount of NIR radiation falling on them, 
regardless of the direction of incidence and distri¬ 
bution pattern of the radiation or whether it is 
brought to a precise focus. b 

The devices discussed here produce an electrical 
signal when flux of radiation falls on them. They 
are of three kinds, namely, photoemissive, photo¬ 
voltaic, and photoconductive. 

The photoemissive type is represented by vacuum 
and gas-filled phototubes and photomultipliers. In 
these, the main effect of the radiation can be re¬ 
garded as the production of an electric current in a 
device with an input impedance of hundreds or 
thousands of megohms. Phototubes with cathodes 
sensitive to NIR radiation have been commercially 
available for many years, and no further develop¬ 
ments in this field were undertaken by NDRC. 
While some photomultipliers sensitive to NIR radia¬ 
tion have been manufactured commercially, they 
were not commonly available before the war. Con¬ 
sequently, work was undertaken under NDRC 
auspices to develop NIR photomultipliers with im¬ 
proved response characteristics and in the sizes and 
designs needed for specific equipments being devel¬ 
oped for military purposes. 

Comparative studies of phototubes were made by 
various NDRC groups in the course of other work; 
some are mentioned in Section 3.2. 

The photovoltaic, photoemf, or barrier-layer cell 
is also in common use in light meters, footcandle 
meters, etc. In such a cell, the main effect of radia¬ 
tion is the production of a small voltage in a 

a Northwestern University, Evanston, Ill. Now at the 
University of Chicago, Chicago, Ill. 

b Image-forming NIR receivers, such as metascopes, elec¬ 
tron telescopes, and dissector tubes developed under NDRC, 
are treated in the Summary Technical Report, Division 16, 
Volume 4. 


low-resistance device. This effect is ill suited for 
electronic amplification, unless the excellent thermo¬ 
couple circuits developed under NDRC during the 
war (see Chapter 8) could be applied to the prob¬ 
lem. No NIR photovoltaic development was under¬ 
taken very seriously by NDRC, although some 
unstable photoemf cells were produced in the course 
of the work described in Section 3.3.1. 

The great NDRC emphasis was on the third kind 
of cell, namely, the photoconductive, with six con¬ 
tracts eventually devoted to the study and develop¬ 
ment of manufacturing techniques for such cells, 
as described in Section 3.3. In addition many other 
contracts applied them as detectors in communica¬ 
tion and signaling systems (Chapters 4 and 5) and 
heat-detection systems (Chapter 9). No photocon¬ 
ductive cells were commercially available when this 
work was started, but more than ten thousand of 
them were produced in consequence of the work 
described here. In these cells the principal effect of 
radiation may be described as a change of resistance 
which is associated with a change of potential dif¬ 
ference or current if a current has been flowing 
previously. They can be made with resistances of 
the order of 10 megohms, large enough to work well 
with amplifiers but small enough so that they are 
not as much affected by humidity and surface leak¬ 
ages as the much higher resistance phototubes. 

The two most successful photoconductive types 
made were the Cashman-type thallous sulfide [TF] 
cells, responsive in the NIR from the visible region 
out to wavelengths of 1.4 (Section 3.3.1), and the 
lead sulfide [PbS] cells, responsive in the NIR and 
beyond in the intermediate infrared [HR] out to 
3.6 n (Section 3.3.2). The TF cell has a respon- 
sivity in the NIR equal to or better than that of the 
best NIR-sensitive phototubes. Its relative infrared 
response, as measured by the standard effective 
holotransmission, ehT° (see Chapter 2 and Appen¬ 
dix), of common NIR-transmitting filters, with 
respect to a standard tungsten source at 2848 K 
color temperature, is much higher than that of the 
available phototubes. 

The PbS cell is not so responsive unless it is 


55 




56 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


cooled to solid C0 2 temperatures or below, but its 
long wavelength threshold makes it an excellent and 
promising detector of thermal radiators whose tem¬ 
perature is only a few degrees above that of their 
background. In one preliminary test 11 a dry-ice- 
cooled PbS-cell system was found to detect thermal 
radiation in the HR from a ship at up to half the 
range obtained with the intensively developed far 
infrared [FIR] bolometer detectors (Chapter 8). 
The speed of response of the PbS cell is much greater 
than that of the bolometers. For a given amount of 
energy in its most sensitive region, below about 3 p, 
the PbS cell is of the order of 100 times more sensi¬ 
tive than thermal detectors, and it will have many 
scientific uses in this wavelength range. 

A number of theoretical problems have been 
raised by the TF and PbS cells, as discussed in 
Section 3.3.1. Much of the theory developed is 
applicable to photodetectors generally, such as the 
discussion of signal, noise, frequency, area, back¬ 
ground light, etc., but it is presented with the TF- 
cell discussion in Section 3.3.1 because it was of the 
greatest interest in its application to this cell. Some 
techniques of test and measurement presented there 
have a wider application, too. 

Both silicon and selenium photoconductive cells 
were also the subject of studies by NDRC (see Sec¬ 
tions 3.3.3 and 3.3.4), but did not yield such fruitful 
practical results. 

3,1,2 Military Applications of NIR and 
HR Detectors 

The detectors described here may be used in at 
least three different kinds of military devices: infra¬ 
red communication and signaling systems, infrared 
radar systems, and heat-detecting devices. 

Any NIR or HR detector can be used in a signal¬ 
ing system to indicate the flashing of some beacon 
which is covered with a suitable filter to give visual 
security. Detection of modulated light, however, re¬ 
quires short response times. With the exception of 
the selenium cell, the detectors described here have 
response times short enough to detect modulated 
NIR radiation up to 2,000 cycles (Chapters 4 and 
5). They consequently are excellent detectors for 
voice or code communication. 

This kind of infrared signaling or communication 
has the military advantage that it can be secret and 
horizon-limited. It can come from a narrow-angle 


source so filtered that it is detectable only by other 
NIR or HR detectors of exactly the right kind 
pointed in exactly the right direction (Section 
4.1.2). These detectors therefore make available 
for military use two new, secure communication 
channels, the NIR and the IIR (see Section 4.8), 
which can be used under combat conditions requir¬ 
ing radio and radar silence. 

For infrared radar systems, where the cells are 
used for detecting NIR reflections from mirrors or 
diffuse targets, cells with very short response times, 
a few microseconds or less, are required. Such re¬ 
sponse times are obtained only with vacuum photo¬ 
tubes and multipliers. The multipliers discussed 
here were used in an infrared radar [IRRAD] sys¬ 
tem described in Chapter 6. 

For thermal detection the requirement on cells is 
a long wavelength threshold in the IIR. Such 
thresholds are possessed only by the PbS cell and 
some related cells as yet little developed. These cells 
may be used to locate and track warm military ob¬ 
jects such as men, guns, tanks, ships, planes, motor 
exhausts, etc. A comparison of PbS cells with bolom¬ 
eters was given above. Evidently the PbS-cell 
sensitivity to the radiation from the target is com¬ 
parable with that of bolometers and may become 
better with further development, even though only 
a small fraction of the total radiation emitted by 
the target can be utilized by the PbS cell because 
of the limited region of spectral response (Section 
4.8). The speed of response of the PbS cell com¬ 
pared to bolometers may be a great military asset. 

All of these detector functions have been utilized 
in World War II. German, Italian, and Japanese 
voice infrared communication systems saw field 
use; British, French, and American systems were 
developed and at least put into production (Chap¬ 
ter 4). The Germans used PbS cells for detecting 
ship targets at night in the English Channel and in 
experimental proximity fuzes. 

It seems likely that the NIR-IIR communication 
function will grow in military interest, and that the 
combined heat-detection, homing-bomb, guiding, 
and tracker problems may rise to supreme impor¬ 
tance. 

3 2 PHOTOTUBES AND PHOTOMULTIPLIERS 

Almost all of the NDRC work on NIR-sensitive 
photoemissive cells was carried out under Farns- 







PHOTOTUBES AND PHOTOMULTIPLIERS 


57 


worth Television and Radio Corporation Contract 
OEMsr-1094. This contract was principally devoted 
to the manufacture of special multipliers, but a few 
special infrared-sensitive vacuum and gas-filled 
phototubes needed by other NDRC contracts were 
also constructed under this contract as a matter of 
expedience. 

Special NDRC development programs on vacuum 
and gas-filled infrared-sensitive phototubes were 
not undertaken, because it was felt that these were 
already well developed commercially and were 
widely available with enough different sizes and 
characteristics (see any RCA Tube Handbook, for 
example) to fulfill almost any foreseeable military 
need. 

With infrared-sensitive multipliers, the case was 
different. The cesium and silver oxide (type Si) 
surfaces needed to produce high-infrared respon- 
sivity seem to be more difficult to manufacture than 
blue-sensitive surfaces. The few commercial NIR 
multiplier types which had been previously in pro¬ 
duction, by Western Electric Company, Farnsworth, 
and RCA, had been discontinued by the time the 
war started. The contract with Farnsworth for mul¬ 
tipliers was consequently initiated in June 1943, so 
that fast-response-time NIR detectors could be ob¬ 
tained which would be suitable for both audio¬ 
frequency and radio-frequency NIR communication 
systems (Chapter 4), for infrared radar (Chapter 
6), etc. 

Although no other NIR phototube development 
was undertaken by NDRC, extensive comparative 
measurements of many tubes and photoconductive 
cells were made by University of Michigan Con¬ 
tract NDCrc-185 and by Northwestern University 
Contract OEMsr-990. In addition some phototube 
circuit developments of especial value for voice 
communication reception were achieved by the lat¬ 
ter contract, as reported in Section 4.4.2. 30 Both 
contracts, together with Northwestern University 
Contract OEMsr-235, also investigated enemy 
phototube characteristics, as mentioned in Section 
4.3. 

Additional studies of the operating characteristics 
of the Farnsworth multipliers were made by West¬ 
ern Electric Company Contract NDCrc-185 in con¬ 
nection with the IRRAD work and are reported in 
Chapter 6. 

Some other work on photoemissive devices de¬ 
serves brief mention at this point. This was the 


study of NIR photoemission from alkali blacks by 
Johns Hopkins University Contract OEMsr-610 in 
1943-44, under Project Control AC-63, as a side 
line from some other work they had been doing. 

By evaporating the metals at low air pressure 
rather than in vacuum, it was found that sodium, 
potassium, cadmium, and tellurium “blacks” 
(oxides) might be deposited on a glass plate. These 
surfaces had considerably longer wavelength thresh¬ 
olds than the pure metal evaporated in vacuum, 
9500 A for sodium black, for example, as compared 
with 5900 A for metallic sodium. No permanently 
sensitive cells were prepared, and the most promis¬ 
ing cell, formed by a deposit of tellurium black with 
a maximum spectral response near 1.6 p, was less 
sensitive than an old Case Thalofide cell (Section 
3.3.3) by a factor of about 200. The study was 
therefore not continued. 

Course of Development 

Farnsworth Contract OEMsr-1094 was initiated 
in June 1943 to study the general optical and elec¬ 
trical characteristics of photomultiplier tubes and 
to develop and construct such tubes for specific 
purposes, emphasizing wide angle of view, good 
infrared responsivity, and high signal-to-noise 
ratio. 21 * 22 ’ 31 

The contract started under Project Control 
NS-159, concerned with ship-to-ship communica¬ 
tion. Later NR-103 and CE-22, concerned with 
IRRAD, were applied when it appeared that those 
executing the original contract would be devoting 
most of their time to the development of special 
photomultipliers for IRRAD. 

In the course of the work the contract built some 
130 6-stage (6PE series) end-view multipliers, 
mainly for use by various NIR communication con¬ 
tracts and for NIR laboratory photometry; about 
70 14-stage tubes (14PE series), 6 11-stage tubes 
(11PE series), and 90 10-stage tubes, for use in 
IRRAD systems; and some 70 vacuum and gas- 
filled NIR phototubes of special design which were 
made for various other contracts for communication 
studies and other purposes. 

The experimental work of the contract was ter¬ 
minated in August 1945. 

Details of Construction and Performance 

Some general characteristics of multipliers may 
be brought to mind. The signal response is inde- 



58 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


pendent of frequency up into the megacycle region. 
Noise is largely “shot noise” and noise from leakage 
currents, and has a white spectrum independent of 
frequency for constant bandwidth. It is well known 
that for such a spectrum the rms noise is propor¬ 
tional to the square root of the bandwidth. These 
characteristics are quite different from those of the 
photoconductive cells to be discussed later. 

All of the tubes made had cesium-silver-oxide 
type Si cathode surfaces. A typical spectral-response 
curve for such a surface is shown in Figure 7. The 
curves are very variable, as the cleaning, oxidation, 
and silvering of the cesium surfaces, which are the 
processes important in obtaining high infrared re¬ 
sponse, cannot be carried out reproducibly. Relative 
NIR response on each Farnsworth tube was deter¬ 
mined from the ehT° of a Polaroid Corporation 
XR3X48 infrared-transmitting filter (see Chapter 
2) with respect to the tube in question. The ehT° 
values obtained range from 5 to 40 per cent, median 
about 20 per cent. The values seem to be related to 
the absolute responsivity of the tubes or to any 
other parameter of cell construction. 

In the 1944 work, the cesium for the surfaces was 
introduced into the tubes by distillation from glass 
capsules which were opened when the tube was 
being formed. Later, in the 6-stage tubes, the cesium 
was developed from pellets containing cesium 
chromate and a reducer. These were placed in a 
side tube which could be heated or cooled to regu¬ 
late the amount of cesium. 

The thermionic current from the surface being 
formed was measured during evaporation to indi¬ 
cate when the layer had reached the correct thick¬ 
ness. An attempt was made to govern the thickness 
by evaporating until maximum infrared sensitivity 
was obtained, but tubes so formed were invariably 
very noisy, so the thermionic indication was used 
instead. 

Six-Stage End-View Multipliers. The 6-stage 
tubes were similar to that shown in Figure 1, 
except for variations in bulb shape and socket 
design. The tubes had a dish-shaped cathode, 
six multiplying boxes, each about % 6 x% 6 x 1 / 4 inch 
in size, and a collector plate. The usual cathode 
diameter was % inch, the external tube diameter 
1% to 1% inches, and the tube length 3 or 4 inches. 
The overall voltage could be from 1,500 to 3,000 
volts, depending on the tube. About 0.1 watt could 
be safely dissipated in the collector. 


Out of 64 6PEA tubes built, 36 were considered 
usable. The responsivities of these varied from 40 
to 440 ma per hololumen. 

The response was not very uniform across the 
cathode surface, because of the poor electron focus¬ 
ing which resulted from designs having the first 
multiplier stage at one edge of a large cathode. 
Noise was not measured. 

On these first tubes, the end window was made 
flat in order to give a wide angle of view, but the 
window apparently accumulated charges which re¬ 
duced the response at low light levels. 



Figure 1 . Six-stage multiplier. 

The signal-to-noise ratio [S/N] was improved by 
using a hemispherical window in Type 6PEB. About 
30 tubes out of 50 of this type were usable, with re¬ 
sponsivities from 30 to 1,500 ilia per hololumen. The 
signal equivalent of noise varied from below 
5 X 10‘ 9 up to 10~ 7 him, reduced to a 25-cycle 
bandwidth. 

Twelve 6PEC tubes were made with a new base. 
Six were usable. Responsivity was 70 to 500 ma per 
hololumen, and the noise equivalent was between 
2X10" 8 and 10~ 7 him at 25-cycle width. 

Fourteen-Stage Tubes. Several 14-stage multi¬ 
pliers, to fit into a 2%-inch shell, were built for the 
triple-mirror form of IRRAD (Chapter 6) devel¬ 
oped by Western Electric under Contract OEMsr- 
1267. A high multiplication factor and a high out¬ 
put current were wanted so as to reduce the need for 
thermionic amplification. 

These requirements necessitated larger multiply¬ 
ing stages than had been used previously, the omis¬ 
sion of the side walls on these stages, and a construc¬ 
tion in which all the leads are carried directly 
through the glass walls by individual seals. 

The type finally arrived at is 7 inches long and 



PHOTOTUBES AND PHOTOMULTIPLIERS 


59 


1% inches in diameter, with a cathode area of %xl 
inch, and with all stages 1 inch long. Thirty-two 
tubes of the final type were made of which 14 were 
usable. Overall voltages were 1,700 to 2,500, respon- 
sivity 10 to 200 amp per hololumen; the noise 
equivalent was 3 X 10~ 10 to 2 X 10" 8 him at 25-cycle 
bandwidth. 

This final type, 14PEI, was arrived at only after 
construction of some 35 intermediate and experi¬ 
mental types to find the best design. In the first 
tubes built the current output choked up (saturated) 
at voltages as low as 900 volts in some cases, and 
was not increased by raising the voltage. This situa¬ 
tion was improved by omitting side walls on the 
later stages and making them larger. Still more 
improvement was obtained by bringing the leads 
out of the side of the tube (see Figure 3), as it was 
found that they had been distorting the dynode 
fields when they were taken out at the base. An¬ 
other solution, found successful on one experimental 
tube, is to leave the leads inside the tube, bringing 
them out of the base as usual but enclosing them 
inside the tube in individual glass sheaths. 

Eleven-Stage Multipliers. These tubes were con¬ 
structed 4or use in the diffuse-reflecting IRRAD 
equipment assembled by the University of Michigan 
under Contract NDCrc-185. The final tube, 11PEJ, 
is 2 Ys inches in diameter and 8 inches long, with 
%xl%-inch cathode area. All stages are 1% inches 
long. The leads are brought out the side. 

Six tubes were made, 4 usable. Operating voltages 
were from 2,000 to 2,400; responsivity, 3 to 70 amp 
per hololumen; noise, 10" 9 to 5 X 10' 8 for 25 cycles. 

Ten-Stage Multipliers. For a modified IRRAD 
system being developed by Bell Telephone Labora¬ 
tories under Contract OEMsr-1267 for Navy use, 
a 10-stage tube was worked out which was designed 
to feed into a thermionic amplifier. It was to have a 
full-cone angle of acceptance of 80 degrees and 
cathode area 0.3x0.5 inches. Some 90 tubes of 
various designs were constructed in attempting to 
meet these requirements and the stringent restric¬ 
tions on overall size and weight, but few of the tubes 
were very good and the final design was not yet 
fixed at the termination of the contract. 

The first type made was a modification of the 
successful 14-stage type, but inconvenient restric¬ 
tions were placed on tube diameter and lead loca¬ 
tion by the mechanical requirements, and no tubes 
were satisfactory. Another type had a translucent 


cathode, which permits an easy fulfillment of the 
optical requirements, but the one tube made turned 
out to be poor. 

A number of side-view tubes were made. Special 
techniques for making glass molds for lead-ins were 
worked out so as to reduce fluorescence, which had 
been found to increase the noise level. Metal, glass, 
and mica screens were tried between stages, and 
sharp points on lead wires were eliminated in an 
attempt to avoid corona discharge. Stages were 
made with and without side walls. New cathode 
shapes were developed to improve the electrostatic 
collection of the primary electrons. 

It was believed at the end of the contract that 
most of the problems of this 10-stage system had 
been solved and that a final design could then have 
been quickly evolved and produced on a large scale 
if desired. Some 12 tubes out of the 90 built during 
this study were regarded as fairly satisfactory. 
Optimum voltages on these ranged from 1,400 to 
3,200; the sensitivity was from 0.4 to 6 amp per holo¬ 
lumen; and the noise equivalent was from 3 X 10“ 9 
to 10 -7 him for 25 cycles. 

Special Tubes. A number of special multiplier 
tubes were made and designs for others were worked 
out under Contract OEMsr-1094. Among these were 
tubes with the first stage operating through a hole 
in the center of the cathode, which gave more uni¬ 
form sensitivity over the cathode surface than with 


MIRROR 



Figure 2. Built-in reflection-optical system. 


the first stage at the side as in the usual design. 
Other tubes were designed like that shown in Fig¬ 
ure 2, with a built-in reflection-optical system. A 
14-stage multiplier having grid or mesh stages was 
designed; this form is very economical of space, 
since the electrons travel straight down the tubes 
without complicated focusing structures and paths. 
The 30-stage tube shown in Figure 3 was built for 










60 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 



Figure 3. Thirty-stage multiplier. 


finding the optimum voltage distribution among the 
dynodes. 

One of the most interesting developments was 
that of a quadrant multiplier, each quadrant having 
6 stages, as shown in Figure 4. These multipliers 
were built for Douglas Aircraft Company in order 
to detect radiation levels of 50 to 100 microlumens 
from distant sources presumably for use in tracking 



Figure 4. Quadrant multiplier. 

equipment to follow such sources. In tracking, the 
differential output from opposite quadrants actuates 
servo mechanisms which keep the optical system 
pointed toward a target so that the target image 
stays almost centered on the origin of the quadrant 
coordinate system at all times. 

In these tubes all of the quadrants are electrically 
connected, the four first stages together, the four 


second stages together, and so on except for the four 
collectors whose leads are brought out separately. 
The cathode is dish shaped, with the quadrants 
separated by “fences.” The cathode diameter is 1 % 
inches. The tube diameter is 2 a /2 inches. The operat¬ 
ing voltage is 1,200. The sensitivity is of the order 
of 200 ma per hololumen and the dark current is 
equivalent to that produced by 100 phlm of radia¬ 
tion. Thirty-three tubes were built, 16 usable. 

In addition, 23 NIR-sensitive vacuum photo¬ 
tubes with very large cathodes, 3x8 inches, were 
built for an Army contract. Sixteen were usable, 
with sensitivities between 20 and 33 pa per holo¬ 
lumen for tungsten light. Also some 40 NIR- 
sensitive vacuum phototubes, about 10 usable, were 
built for BuShips under a Polaroid Corporation 
contract. In these the cathode was evaporated on the 
glass bottom of a tube shaped like an Erlenmeyer 
flask 5 inches in diameter. These tubes were to be 
used at the focus of a mirror, supported by the neck 
of the flask, which passed through the vertex of the 
mirror. The responsivities were about 30 pa per 
hololumen. 

Six gas-filled NIR phototubes, 2 usable, were 
made with 2x4-inch cathodes for Northwestern 
University under Contract OEMsr-990. Respon¬ 
sivities were 185 and 560 pa per hololumen at 100 
volts with 0.01 him of incident flux. 

Fundamental Studies 

A number of fundamental studies were carried 
out in connection with the multipliers. 

One involved the demonstration that if angles 
of acceptance of about 90 degrees or over are re- 




PHOTOCONDUCTIVE CELLS 


61 


quired, no optical system can give any substantial 
improvement in response, and the tube is best used 
without one. 21 ’ 31 This is a special case of the gen¬ 
eral angle-of-view theorem discussed in Section 
4.1.3. 30a 

Another study concerned the optimum voltage per 
stage of a multiplier. 21 Assuming for simplicity that 
the multiplication per stage is proportional to the 
square root of the voltage, it was shown that the 
maximum overall gain with a large number of stages 
and fixed total voltage is obtained when the multi¬ 
plication factor is Ve (base of natural logarithms) 
or 1.65; this means a much higher number of stages 
than are customarily used. This conclusion was 
checked with the 1-foot, 30-stage multiplier shown 
in Figure 3 and found to be correct as long as the 
output signal is small. 

Other discussions have been given 21 of the influ¬ 
ence of load resistor and space charge on the avail¬ 
able output, and of the proper division of amplifica¬ 
tion between a multiplier and a following thermionic 
amplifier. 

Several devices were constructed or assembled for 
measuring sensitivity and noise of tubes. They in¬ 
cluded a modified Macbeth illuminometer, a noise 
box similar in principle to the photocell test set 
(Section 3.3.1), and equipment for measuring noise 
from oscillograph traces. 

Present Status 

Farnsworth Contract OEMsr-1094 was termi¬ 
nated in August 1945. Some further needed work on 
photomultipliers for military use is presumably 
being carried out there under a succeeding Navy 
(BuShips) contract with Farnsworth. 

Recommendations 

The achievements made under the contract in 
improving multiplier design and performance were 
considerable and should not be lightly dismissed, 
since the techniques involved are very exacting 
and difficult to reproduce in large-scale produc¬ 
tion. 

It is felt that the Armed Services ought to main¬ 
tain educational orders in NIR phototubes and 
photomultiplier tubes of various types at all times 
so that a background of developmental and manu¬ 
facturing experience may be built up and suitable 
types of tubes be made available when needed. 


3 3 PHOTOCONDUCTIVE CELLS 

331 Cashman Thallous Sulfide [TF] 

Cells 

The other type of selective photosensitive detector 
intensively studied by NDRC was the photoconduc- 
tive cell. Because of its great undeveloped possibili¬ 
ties, the emphasis on this type of cell was strong 
from the beginning and became much stronger after 
the Cashman TF cell began to be made successfully. 
Eventually more of the NDRC work was devoted 
to the TF cell than to all of the other photodetectors 
together. 

Though thousands of papers and hundreds of pat¬ 
ents have appeared on photoconductive cells in the 
last 50 years, few cells have been commercially 
marketed for the reason that most of the early ones 
were unstable when subjected to voltage or strong 
light. This problem has at last been solved and the 
way to large-scale commercial manufacture of the 
TF cell and related types is now open. 

Initial State of the Art 

Case Cells. In 1917, T. W. Case discovered photo¬ 
conductivity in a thallium-sulfur compound con¬ 
taining oxygen. 1 * 3 Cells made from this compound 
were early turned to military applications in the 
voice and code signaling and communication systems 
described in Section 4.2.2. They were marketed 
commercially during the 1920’s as Case Thalofide 
cells. 

In making a cell, thallous sulfide which had been 
previously heat-treated was smeared in a layer on 
a quartz disk in air at elevated temperatures. Since 
the resistivity of the material is high, the resistance 
of the cell was reduced by shaping the contacts into 
coarse comblike grid patterns intermeshing with 
each other. These contacts were of lead sprayed 
through a stencil with a metal-coating pistol. 

The measure of sensitivity of the cells was the 
change in resistance produced by exposure to a 
known illumination level from a tungsten lamp oper¬ 
ating at an unspecified temperature. The original 
resistance of Case cells was 100 to 500 megohms. 
This decreased by a factor of 2 in the average cell 
(type RL) on exposure to 0.25 footcandle. Excep¬ 
tional cells (type SRL) decreased by this factor on 
exposure to only 0.06 footcandle. 

This initial sensitivity was fair and was fre- 



62 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


quently not greatly exceeded by currently produced 
TF cells, but the stability of the cplls was poor. 
They were sealed in red glass and the user was 
instructed not to expose them to more than 0.5 foot- 
candle; exposure to ultraviolet light or even a few 
hours’ exposure to room light might destroy their 
sensitivity permanently. 

The maximum spectral response of these and sim¬ 
ilar cells is near 1 p, with a long wavelength thresh¬ 
old near 1.3 p. Usually a secondary maximum is in¬ 
dicated near 0.5 p but there is some suspicion that 
this is the result of drift and instability and may be 
spurious. 

Other Work. The high NIR response and the in¬ 
complete disclosure of the process of manufacturing 
the Case cell stimulated research on such cells in 
many laboratories. The most extensive work was 
carried out by Majorana and Todesco in Italy; by 
Dubois and Fournier in France, who developed a 
cell later manufactured by the CEMA Company; 
and in Germany by Sewig, whose cells were devel¬ 
oped in the Osram laboratories. 1 - 3 Russian and Jap¬ 
anese developments have also been reported. 

Most of these authors believed the instability to 
be an inherent property of the thallous sulfide cell. 
Their cells were all much like the Case cell. As far 
as the literature reveals, the sensitive material was 
always T1 2 S, containing only slight, if any, excesses 
of T1 or S, and combined with oxygen in an un¬ 
known way. 

German Cells. A few German thallous sulfide 
cells, intended for use in light-beam telephone sys¬ 
tems, were captured during the war and were 
studied by Northwestern University under Contract 
OEMsr-235 and by University of Michigan under 
Contract NDCrc-185. Except for their small size, 
adapted for use in a narrow-beam stationary sys¬ 
tem, and the absence of a conducting grid, they 
appeared to be not very different as to method of 
fabrication from the cells just described. On a com¬ 
parative basis they were certainly no better in over¬ 
all characteristics than Cashman cells being made 
at the time. The sensitive areas were all small, from 
1 to 6 millimeters square, and were mounted on the 
back of a protective red glass filter inside an un¬ 
evacuated plastic capsule. Apparently they had all 
been made some years earlier and had been gen¬ 
erally replaced more recently in German military 
equipment by lead sulfide cells of similar de¬ 
sign. 12 - 13 * 14 


sag 


British Cells. During World War II, the develop¬ 
ment of thallous sulfide cells was also undertaken 
by the British Admiralty Laboratories. The British 
and American programs were kept in step for some 
time by a fairly effective liaison program. The 
British techniques were originally superior, but were 
eventually surpassed in this country. Some of the 
final British photosensitizing processes and cell de¬ 
signs were adapted from the Cashman processes 
and designs. Approximately 650 British cells were 
sent to the University of Michigan under Contract 
NDCrc-185 during 1943 and 1944 for testing prior 
to their being accepted for use by the U. S. Navy. 
Subsequently this contract furnished a photocell 
test set (see Section 3.3.1) and designs for it to the 
British. 

Many of the British cells were excessively noisy, 
and their average S/N ratios never came up to the 
averages finally obtained on cells produced by 
Northwestern University under Contract OEMsr- 
235. 

Course of Development 

The Cashman TF cell is not greatly different in 
basic design from the Case Thalofide cells, but the 
manufacturing techniques have been highly refined 
and standardized. The quality is less variable from 
cell to cell and the individual cells are much more 
stable. The loose confusion of the names of these 
two cell types, which has crept into some reports, 
is to be deplored. 

Military Interest. Two specific military problems 
were associated with the first NDRC interest in 
thallous sulfide cells. The first was the detection of 
night-bombing planes by NIR radiation reflected 
back from them (Reference 39, Chapter 4). The 
second was the need for detection devices in an 
NIR recognition and signaling system (Section 
5.2). Both problems were studied by the Univer¬ 
sity of Michigan under Contract NDCrc-185, under 
Navy Project Control NS-151 and later under SC-5. 

The tremendous enlargement of military interest 
in the TF cells during the course of the work is re¬ 
flected in the numerous additional project control 
numbers eventually assigned to their design, pro¬ 
curement or production, and testing. Number NS- 
225 was concerned with the procurement of British 
cells by BuShips and with the testing of them under 
Contract NDCrc-185. Numbers NS-159 (see Sec¬ 
tion 4.4.2) and AC-226.01 were concerned with in- 





PHOTOCONDUCTIVE CELLS 


63 


frared work in general. Other numbers dealt with 
specific military devices being developed under 
NDRC in which TF cells were used: NA-194 (see 
Section 5.3), AC-101 (Section 5.3), NA-191 (Sec¬ 
tion 5.5), AC-226.03 (Section 4.4.3), and AC-226.04 
(Section 4.4.4). 

Some of these devices used the TF cell as an NIR 
detector for code communication or signaling. One 
used the cell in locating a distant optical unit by 
reflected light (Section 5.4). Another used the cell 
in a positional indicating system (Chapter 7). Sev¬ 
eral devices made use of the cell as a voice detector 
after it was discovered in 1944 that the cell would 
give good S/N ratios at frequencies up to 3,000 
cycles. 

The possible use of thallous sulfide in image-tube 
presentation was investigated, 3 and the sensitivity 
of TF cells as thermal detectors was measured, 10 
but there was not enough effect in either case to 
compete with materials of other kinds already being 
used for these purposes. 

History of Development. The NDRC work on 
thallous sulfide cells stemmed from a program of 
basic research on semiconductors which had been 
begun by R. J. Cashman of Northwestern Uni¬ 
versity in the spring of 1941 and which was pri¬ 
vately resumed by him for a few months in the 
fall of 1941. This work was at first concerned with 
the question of whether semiconductors could be 
evaporated in a vacuum without decomposition, and 
favorable results had been obtained with T1 2 S and 
some other compounds. 

These studies were continued under Contract 
NDCrc-185 at the University of Michigan in the 
summer of 1941, with special emphasis placed on 
attempting to produce stable and sensitive thallous 
sulfide photovoltaic cells. Sensitive cells were ob¬ 
tained, with responsivities of the order of 7,000 pa 
per hololumen, but they were not stable and suit¬ 
able amplifiers for low-resistance cells of this type 
were not then available. 

About this time some tests with a Case Thalofide 
cell, kindly furnished by W. W. Coblentz, showed 
it to have remarkable NIR sensitivity. Attempts 
were therefore made to manufacture some photo- 
conductive cells and after some promising prelim¬ 
inary results had been obtained, Contract OEMsr- 
235 was set up at Northwestern University in 
December 1941 with the principal object of produc¬ 
ing TF cells as sensitive as the Case cell, but with 


greater stability and adapted to mass produc¬ 
tion. 1 ' 2 ' 3 * 26 

A cell was visualized, as a goal to be achieved, 
which would have the following characteristics. 

1. Stability. 

2. High responsivity. 

a. High signal response. 

b. Low noise. 

3. Dark resistance of 10 megohms or less. 

4. Rugged construction. 

a. Nonmicrophonic. 

b. Vibration- and shock-resistant. 

5. Good frequency response. 

6. High relative NIR responsivity. 

The stability and rugged construction were espe¬ 
cially important for military applications. 

It was expected that the stability would be the 
most difficult characteristic to obtain. The initial 
method of attack on this problem involved the study 
and systematic variation of variables in the manu¬ 
facturing process to find the most favorable com¬ 
bination. Almost from the beginning occasional 
stable and sensitive cells were made, but their pro¬ 
duction did not become reproducible for many 
months. 

The photocell test set equipment for testing the 
responsivity and noise of the cells was produced 
in the fall of 1942 by the University of Michigan 
under Contract NDCrc-185. 23 ' 24 ' 25 Much effort was 
spent thereafter in determining the cause and cure 
of high cell noise. 

Supplemental and coordinated fundamental and 
theoretical studies of the cell properties were begun 
in June 1943, under Contract OEMsr-1036 at the 
Massachusetts Institute of Technology, 6 * 7 ' 8 as this 
aspect had previously been somewhat neglected be¬ 
cause of the pressure for cell production. It was 
hoped that these studies, together with the earlier 
work under Contract OEMsr-561 on the less suc¬ 
cessful selenium cell (Section 3.3.3), would throw 
light on the nature of the photoconductive processes 
involved and aid in determining optimum conditions 
for successful TF cell production. These studies 
were continued until June 1945. Some similar inves¬ 
tigations were also undertaken at the University of 
Michigan. 2 * 9 

Consistently good activated thallous sulfide mate¬ 
rial was being made at Northwestern University by 
the beginning of 1944, and the average yield of good 




/ 




64 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


cells was being rapidly improved. The design of the 
cells for use in the signaling system described in 
Section 5.2 was standardized soon thereafter. This 
design, later called type “A,” had a sensitized grid 
area of %x% inch located directly on the inner 
wall surface of the evacuated cell, rather than on a 
separate flat glass plate in the cell which had pre¬ 
viously been considered necessary for the signaling 
application. This simple and nonmicrophonic de¬ 
sign was patterned after some cells which had been 
made by Cashman in 1941 and has the advantage 
that the layer is sensitive to radiation from both 
sides and thus gives a 360-degree, or all-around, 
view. It apparently served as the basis for the design 
of the British ARL type S/T thallous sulfide cell. 

By February 1944, the yield ratio of good cells 
had reached about 90 per cent of the number being 
made. For these type A cells, the reduction of re¬ 


sistance on illumination by 0.25 footcandle of tung¬ 
sten lamp radiation was by factors of 10 to 30 as 
compared with factors of 2 to 8 for Case Thalofide 
cells. The S/N ratios of the type A cells, measured 
on the photocell test set (see below) averaged about 
58 db for 1 phlm chopped 90-cycle unfiltered signal 
from a tungsten lamp operated at 2848 K color 
temperature. A 15-minute exposure to direct sun¬ 
light and a 15-hour polarization test with 22.5 volts 
across the cell in the dark were incorporated as 
standard stability tests on all cells. 

By January 1944 it was felt that the original 
objective of designing a TF cell suitable for pro¬ 
duction had been achieved, and Contract OEMsr- 
1322 was then initiated at GE to set up facilities 
and develop manufacturing techniques for quantity 
production of type A cells. 4 This freed those working 
under Contract OEMsr-235 for the construction of 



Figure 5. TF cell types. 

ng&tm&FD 



PHOTOCONDUCTIYE CELLS 


65 


cells of special design and the investigation of new 
cell materials as described in Section 3.3.2. 

Actual quantity production of type A cells was 
taken over at GE by Navy contract in late 1944. 
Meanwhile the voice reception properties of the TF 
cell had been discovered and TF cells of a larger 
size, 114 x 2 % inches in sensitive area, were being 
used in the voice communication system of Section 
4.4.2. This size was called the type B TF cell and 
was later given the RMA type number 1P38. In 
April 1945, a project for the development of the 
manufacturing techniques for the type B cells was 
begun by RCA under Contract OEMsr-1486. 5 

Various groups, especially those working on the 
military devices described in Chapters 4, 5, and 7, 
have compared the TF cells with different kinds of 
photoemissive cells. Theoretical studies relating 
to the TF cell properties have been made by 
some individuals besides those assigned to this 
work 

By the end of the war, the minimum Navy accept¬ 
ance specification for the type A cell was 48 db S/N 
ratio per microhololumen, as measured on the pho¬ 
tocell test set, with 2.5-millimeter Corning 2540 
filtered signal; it was 46 db for the type B cell. On 
the same test, average values for cells produced at 
Northwestern University were 56 db for type B, 58 
db for type A and 68 db for a large number of 
%x%-inch cells produced for various special pur¬ 
poses. On the same test, a sensitive but rather noisy 
900-megohm Case Thalofide cell with 2-square 
centimeter area gave 21 db S/N ratio, but this fig¬ 
ure may be 20 to 40 db too low because the test set 
is not designed for such high-resistance cells. The 
TF cells were being used quite confidently in ex¬ 
perimental equipments designed for daylight opera¬ 
tion and for continuous exposure to direct sunlight 
(Section 4.4.4). Most cells withstood application of 
90 to 180 volts continuously without objectionable 
noisiness or loss of sensitivity. 

The threshold sensitivity (signal equivalent of 
noise) of the cells is about equal to that of the best 
NIR phototubes in the best circuits developed dur¬ 
ing the war and is 10 to 30 times better than that of 
these phototubes when used in conventional circuits. 

Description of Cells and Production Methods 

Cell Types and Construction. Eleven different TF 
cell types made under Contract OEMsr-235 are 
shown in Figure 5. 


The cells are made by evaporating a few milli¬ 
grams of prepared and preoxidized T1 2 S in vacuum 
onto a comb-shaped intermeshing grid made of 
Aquadag (colloidal carbon) painted on the inner 
wall of the cell or on a glass plate (Figure 6). The 
cell is of Nonex glass. The grid lines are extended so 
that they will make contact at gold-plated tungsten 
lead-ins. In the cells shown in Figure 1, the size of 
the sensitive grid area varies from V^x 1 /^ inch to 
2x3 inches, and could be made still larger. The grid 
spacing is from 15 to 25 lines per inch. 



After evaporation, the T1 2 S layer is sensitized by 
baking for 20 minutes in an oven at about 270 C in 
the presence of about 200 p pressure of oxygen and 
15 p pressure of water vapor. After the cell has 
cooled to room temperature, the remaining gas is 
pumped out to at least a 2-p vacuum and the cell 
sealed off. 

The cells are cemented in spun aluminum four- 



f 












































66 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


prong standard bases having an isolantite insulator 
coated with moisture-repellent lacquer. 

Successful cells can be made only if meticulous 
care is exercised in the purification of the T1 2 S and 
other components and in the cleaning of the glass¬ 
ware. 

Summary of Cell Characteristics. Some proper¬ 
ties of the cells are summarized in Table 1. 


measure of the low-level response of a cell. The 
latter response is best measured on the photocell test 
set. For the standard 1-phlm 90-cycle NIR-filtered 
signal, a type A cell, with matched load resistor and 
90 volts applied across both, gives about 72 db 
signal output and about 14 db noise output in a 
1.8-cycle band-pass, referred to a 0-db level of 1 pv 
rms. The S/N ratio is thus some 58 db. Without the 


Table 1. Properties of some photoconductive cells. 


• 

Property 

Case 

Thalofide 

TF 

14 X14 in. 

Cell 

TF type 

A 

% x % in. 

Type 

TF type 

B 

114x2 in. 

PbS* 

16 x 1 mm 

PbS 

14 x 14 in. 

Area (sq in.) 

0.3 

0.06 

0.6 

4 

0.0008 

0.06 

Grid spacing (in.) 


V 25 

V25 

Vl5 

No grid 

1&5 

R d (megohms) 

300 

19 

6.1 

3.5 

1,20 

1 

St for 10 -6 him filtered (db) 


89 

72 

64.5 

90,91 

55 

Nt for 1.8-c bandwidth (db) 


21 

14 

8.5 

26,3 

8 

S/Nt (db) 

2R 

68 

58 

56 

64,88 

47 






6 X IO - 10 


Signal equiv. of noise filteredt (him) 

t 

4 XlO“ 10 

1.3 X 10- 9 

1.6 X IQ" 9 

4 X IQ" 11 

4 X 10- 9 

* First figure, room temperature; second figure, dry-ice cooled. 

t Signal and noise 

from photocell test set under standard conditions. 


{ Test set not designed for high-resistance cells. 


The resistance of the cell when in total darkness 
(dark resistance) can be controlled by variations 
in manufacture and is usually set to reach a stable 
value of between 0.5 and 20 megohms after a few 
days’ aging. The capacitance of a type A cell is 
about 20 ppf. 

Figure 7 shows the spectral response curve of a 
cell made at an intermediate stage of the develop¬ 
ment, together with the curve for an Si-surface 
phototube for comparison. In later TF cells, the 
threshold was extended to 1.45 p and the response 
curve was almost flat from 1.2 p to below 0.4 p, 
probably being limited on the short wavelength side 
only by the Nonex wall transmission. There is good 
sensitivity to X rays. 

One method of measuring the cell response to 
integrated light over the wavelength range to which 
it is sensitive is to determine the factor by which 
the resistance decreases on exposure to a standard 
flux from a tungsten lamp. For type A TF cells, the 
factor is 10 to 30, with 0.25 footcandle illumination 
level. 

This method involves high intensities, bringing 
in saturation effects, so that it gives only a crude 


Corning 2540 filter (2.4 mm thick) it increases about 
13 db. The 90-cycle signal does not correlate very 
well with the response to steady light but is very 
satisfactory for predicting the response at higher 
frequencies up to about 3,000 cycles. 



Figure 7. TF cell and phototube, Si, spectral respon- 
sivities. 


The signal and noise are both functions of cell 
area as discussed later under “General Properties.” 
In testing, the flux is normally spread over an 


[ 





























PHOTOCONDUCTIVE CELLS 


67 


area of about %-inch diameter on the cell. Theo¬ 
retically, the signal output for a given flux should be 
independent of the spot size into which the flux is 
concentrated, but in practice saturation and grid- 
line effects invalidate this rule as the spot is made 
smaller. 

The voltage signal output as a function of inci¬ 
dent light flux, with the load resistor matched at 
every illumination level, is shown in Figure 8. It is 
linear from the threshold up to about 120 db output 
level, which corresponds to a flux of about 10 


SIGNAL IN DECIBELS (CURVE B) 

90 100 110 120 130 140 150 160 



Figure 8. TF cell response versus incident flux level 
(modulated). 


phlm for a typical cell; the linearity thus ex¬ 
tends over a range of about 10 5 in flux. The upper 
limit depends on applied voltage, load resistor, sig¬ 
nal frequency, and waveform, amplifier perform¬ 
ance, etc., so that the linearity of the fundamental 
photoprocess in the cell itself may extend to even 
higher flux levels. 

The signal and noise are both approximately pro¬ 
portional to the applied voltage and inversely pro¬ 
portional to the frequency (for constant amplifier 
bandwidth) as shown in Figures 9 and 10, so that 
the S/N ratio of TF cells is almost independent 
of either voltage or frequency up to about 3,000 


cycles where these relations begin to fail. The 
important voice frequencies near 1,500 cycles may 
thus be detected with good sensitivity by TF cells. 



Figure 9. TF cell signal and noise as functions of 
frequency. 


The stability of the cells with respect to voltage 
or strong light has already been stated. On exposure 
to strong steady background light, the most obvious 
effects on the cell are great decreases in resistance, 



Figure 10. TF cell S/N ratio as function of fre¬ 
quency. 


in signal, and in noise. Part of the decrease in signal 
and noise is due to the change of resistance and 
resultant mismatching of the original load resistor, 
but even after the load resistor is readjusted to the 


\ 




























































































































































68 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


new conditions, great losses in signal and noise and 
in S/N ratio may still remain. One experiment, de¬ 
scribed in Section 4.4.4, was made on the effects of 
exposure to a background equivalent to about 1,000 
footcandles of light from the overcast north sky. 
The resistance of a type B TF cell changed from 
about 2 to about 0.05 megohm; the loss in S/N 
ratio, even with the load resistor properly adjusted, 
was about 20 db. Although this loss is serious for 
military equipment which might be needed in day¬ 
light work, it is small compared to the losses pro¬ 
duced in phototubes by background light. 



Figure 11. * Resistance of a TF cell versus tempera¬ 
ture. 


The resistance of the TF cell is strongly depend¬ 
ent on temperature, increasing at lower tempera¬ 
tures according to the usual exponential law for 
semiconductors (Figure 11). The S/N ratio is less 
affected. If the load resistor is properly adjusted at 
each temperature, the ratio appears to increase by 
a few decibels on going from room temperature to 
0 C and then to decrease at lower temperatures. 


Much effort was spent by Northwestern Univer¬ 
sity under Contract OEMsr-990 30 and by the 
University of Michigan under Contract NDCrc-185 
on the design of the best circuit for use with a TF 
cell. The resultant designs are described in Sections 
4.4.2 and 5.2. The essentials appear to be the use of 
a cathode-follower in the first stage, which must be 
located no more than a few inches from the cell; 
and the use of a wire-wound or equivalent low 
noise resistor for the load resistor of the cell. Care¬ 
ful electrical shielding may also be needed around 
the cell to prevent pickup from stray fields. With 
such a circuit it is possible to have a photodetecting 
threshold limited only by the inherent noise of the 
TF cell itself. 

Comparison with NIR Photoemissive Detectors. 
Comparisons were made between TF cells and vari¬ 
ous photoemissive devices as NIR detectors by 
several groups, but perhaps the most elaborate were 
those by Northwestern University Contract OEMsr- 
990 reported under “Choice of Photodetector Cell” 
and “Interrelations of Components,” Section 4.4.2. 30 
The conclusion of this contract was that, with the 
best type of circuits tried for each type of detector, 
the average threshold sensitivity of type A TF cells 
at 1,500 cycles was equal to that of the best of 
the many other photoemissive devices tried, namely, 
Continental Electric Company type CE-l-AB gas- 
filled phototubes. This held true with either cesium- 
vapor or continuous-spectrum sources, NIR filtered 
or unfiltered, within 3 or 4 db one way or the other. 

The conclusions of this contract do not agree with 
those of any other group, as they show the cesium- 
surface phototube in an unusually favorable light. 
This is probably the result of the unusual proper¬ 
ties of the phototube circuit developed by this con¬ 
tract, which has no external load resistor, using 
instead only the grid-to-cathode impedance of the 
first preamplifier stdge to produce a voltage drop 
when the photocurrent flows. This circuit was more 
sensitive by a factor of 10 or more than the con¬ 
ventional circuits which were tried, and appeared 
to have a threshold limited only by the inherent 
dark current shot noise of the phototube itself. 
Those working under other contracts, such as 
OEMsr-1073 of the University of California (see 
“Receiver” in Section 4.3.2) gained an advantage of 
20 to 30 db in changing from phototubes to TF 
cells in a voice communication system. Northwestern 
University under Contract OEMsr-235 found the 


























































PHOTOCONDUCTIVE CELLS 


69 


gas-filled CE-l-AB phototubes mentioned above, 
when used with a 10-megohm load resistor, gave 10 
to 40 db lower S/N ratio than the final type A TF 
cells, both types being measured on the photocell 
test set at 90 cycles with unfiltered tungsten light. 3 

The TF cells have other advantages over a cesium 
surface phototube. One is the higher ehT° values 
for a given NIR filter as a result of the longer wave- 
length threshold (see Figure 7 and Chapter 2). An¬ 
other is the all-around view of the TF cell, with its 
layer formed on the glass cell wall where it can 
receive light from both sides. Still another is the 
complete absence of microphonics, the TF cell hav¬ 
ing no internal parts between which relative motion 
can occur. Also, the TF cell is comparatively in¬ 
different to humidity and moisture, since surface 
leakage resistances of the order of 20 megohms or 
more, which would form a serious shunt across the 
best photocells, have scarcely any effect on TF cell 
output. This was one of the main reasons for aban¬ 
doning phototubes in the voice systems described in 
Sections 4.3.2 and 4.4.2. 

The TF cell output, with properly matched load 
resistor, is seriously affected by background light 
but not so seriously as the output of a phototube, so 
that the TF cell is better for use as a detector in 
equipment which must operate in daylight. Still an¬ 
other advantage is to be found in the reproducibility 
of these desirable characteristics in TF cells, which 
can be constructed on a large scale with a mean 
deviation of about =1=4 db in filtered S/N ratio, 
compared to two or more times this deviation for 
phototube production. Great variations in size of the 
TF cells—from 0.06 to 4 square inches in the group 
shown in Figure 5—can be made without alteration 
of the cell properties, as these are unrelated to the 
size of the surface. 

Disadvantages of TF cells for some purposes will 
be found in their nonlinearity at high light levels, 
and in the strange behavior of signal and noise with 
frequency and background light, especially the loss 
of threshold sensitivity at frequencies over about 
4,000 cycles. 

Details of Manufacture 

Construction of Cell Bodies. The cell is made of 
Nonex so that it will stand the 450 to 500 C tem¬ 
peratures of the T1 2 S evaporation and so that it can 
be sealed directly to tungsten lead-ins. The type A 
envelope is made of selected 1-inch diameter Nonex 


tubing with walls 20 to 40 mils thick. Other glasses 
and lead-ins could be used if tubing of the proper 
sizes were available. Tubing is used instead of 
blown bulbs, because the bulbs obtainable during 
the war show r ed white deposits of B 2 0 3 during work¬ 
ing of the glass. The T1 2 S appeared to react un¬ 
favorably with this or some other constituent to 
form relatively insensitive brown nonuniform 
streaks over the walls and grid. These difficulties for 
some reason did not appear when tubing was 
used. 

The grid lines may be drawn with a short ruling 
pen. The glass surface beneath the grid must have 
no surface cracks or striations into which the Aqua- 
dag might be drawn by capillarity to produce high 
local electric fields and noisiness in the cell. 

Grids have been made not only with Aquadag but 
also with evaporated platinum and gold, with plati¬ 
num, gold, and silver ceramic inks, and with a soft 
lead pencil. The metals occasionally reacted with 
the TBS and so their use was discontinued; the lead 
pencil grids gave noisy cells. The Aquadag was 
therefore chosen as the best material. 

Some observations at Northwestern and MIT in¬ 
dicated that the Aquadag may play an essential 
catalytic role in the oxidation-sensitizing of the 
T1 2 S layer, producing cells more sensitive but noisier 
than those made with metal grid lines. To keep the 
sensitivity but reduce the noise, some cells were 
made at RCA with Aquadag ruled over a metal 
grid; 5 their performance appeared promising but 
there was insufficient time to carry this study to 
completion. 

A few cells have been made at Northwestern with 
multiple grids (two or four) in one envelope. Such 
cells may be useful in tracker systems (see Section 
4.1.3 and “Special Tubes,” Section 3.2). 

The gold plating over the tungsten lead-ins is 
needed to make a good contact to the Aquadag; if 
it is omitted, the cell is more likely to be noisy. 

The TF cell lead-ins are made of 40-mil tungsten 
rod, % inch long, butt-welded to an 8-strand copper 
and nickel braid. The heavy lead-in is needed only 
to give a large contact surface with the Aquadag. 

Quite exacting routines have been worked out for 
the glass-working, cleaning, gold-plating, and ruling 
of grids, in order to avoid and eliminate contami¬ 
nants, deposits, or blisters, which might cause open 
circuits, shorts, poor contacts, or noisy cells, or 
which might react with the T1 2 S unfavorably. For 






70 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


complete details reference should be made to the 
contractors’ final report on TF cell development. 3 

Preparation of Tl 2 S. Highly purified thallous 
nitrate or sulfate is used as the starting material. 
This salt is converted to black amorphous T1 2 S by 
bubbling H 2 S through an ammoniacal solution of 
the salt in conductivity water. After washing and 
filtering the precipitate, it is transferred to a 
vacuum desiccator, air being admitted after the 
second and fourth days just long enough to break up 
the chunks of T1 2 S to promote faster drying. After 
two weeks’ drying, the material is allowed to stand 
in air of about 50 per cent relative humidity for at 
least 72 hours, the moisture catalyzing a slow oxida¬ 
tion. 

Samples of approximately 30 grams are then fused 
in vacuum. Two phases appear, the lighter and more 
volatile being a yellow or red insulating layer which 
is some kind of oxide of the material, probably 
largely T1 2 S0 2 . The denser phase has, on cooling, 
a crystalline metallic appearance and is T1 2 S within 
the limits of error of chemical analysis, but from its 
behavior toward photosensitizing it must contain 
appreciably more oxygen than T1 2 S prepared by 
synthesis from the redistilled pure elements in 
vacuum. The lighter phase is scraped off from the 
denser slug with a knife and discarded; the slug is 
crushed and stored in evacuated ampoules until 
needed for cell fabrication. Fifteen to 50 mg of the 
crushed T1 2 S are used in each cell, depending on the 
cell size. 

Study of Variables in Preparation of Tl 2 S and 
Sensitizing. The production of stable and sensitive 
TF cells was only achieved by Contract OEMsr- 
235 after months of study through 1942 and 1943 of 
many hundreds of systematic variations of manu¬ 
facturing technique and of the effect of these varia¬ 
tions on the cell characteristics. The variables were 
in two classes, those which concerned methods of 
preparation of the T1 2 S starting material and those 
which concerned sensitization of the layer after 
evaporation. They included 

1. Thallous sulfide. 

a. Excess or deficit of metallic constituent 
from stoichiometric proportions. 

b. Impurities. 

c. Crystal size and orientation on receiving 
surface. 

d. Absorbed and adsorbed vapors and gases. 


2. Cell structure. 

a. Contacts (grid). 

b. Receiving surface. 

3. Oxidation. 

a. Oxidation of material before forming sur¬ 
face. 

b. Oxidation of material during or after form¬ 
ing surface. 

c. Effect of vacuum conditions (presence or 
absence of vapors and gases) on oxidation. 

d. Temperature. 

e. Time. 

Previous literature on semiconductors such as 
phosphors pointed to the need of very pure starting 
materials and to the importance of the study of im¬ 
purities. So the first variations were in methods of 
making T1 2 S. It was prepared by precipitation from 
a solution, followed by oxidation by moist air (the 
final method adopted). It was prepared by precipi¬ 
tation without oxidation. It was prepared by syn¬ 
thesis in vacuum from the elements, both with 
stoichiometric proportions and with regularly varied 
excesses of one element or the other. It was prepared 
by using elements purified by multiple distillation 
in vacuum; elements only commercially pure; and 
elements with known amounts of lead, copper, or 
silver impurity. More than 40 samples alone were 
prepared under Contract OEMsr-235 by synthesis 
in vacuum. At MIT, under Contract OEMsr-1036, 
thallium was also prepared electrolytically; T1 2 S 
samples were made with total impurities of less 
than 15 parts per million. In some cases all opera¬ 
tions were carried out in an atmosphere of inert 
nitrogen. 

The same exhaustive attention was given to every 
stage of cell manufacture. For the detailed list of 
the variations and of their effects on cell properties, 
reference must be made to the original reports. 3 * 6 
Some of the effects, especially those produced by 
impurities, throw light on the theory of the photo- 
conductive process but cannot be discussed in detail 
here. 

The general results and conclusions were as fol¬ 
lows: 

For good cells, the metallic impurity had to be 
less than 0.5 per cent and high purity was prefer¬ 
able. The T1 2 S, however it was made, had to be 
prefused in vacuum before introduction into the cell 


RE&W*iST-ED 



PHOTOCONDUCTIVE CELLS 


71 


in order to avoid eruptive evolution of gas and con¬ 
sequent nonuniformity in the evaporated layer. 

Material prepared either by vacuum synthesis or 
by precipitation from solution without subsequent 
oxidation gave much less sensitive cells than mate¬ 
rial which had had some exposure to air. The first 
excellent cells were obtained from some precipitated 
T1 2 S which had been inadvertently exposed to air 
in a leaky desiccator for several months. 

Preoxidized samples required less oxidation-sen¬ 
sitizing after evaporation of the layer in the cell; a 
lower baking temperature sensitization was possible, 
and this was associated with lower cell resistance 
and better stability. The preoxidized samples evi¬ 
dently contained a trace of some substance impor¬ 
tant in their photoconductivity, even though chemi¬ 
cal analysis showed them to be pure T1 2 S within 
the limits of error. Several experiments showed that 
this trace substance was indeed oxygen. 

The best and simplest method of cell preparation 
was as follows: The glass cell blank, with the grid 
ruled and the end sealed off, was baked out in a 
furnace. The crushed T1 2 S was introduced and the 
cell sealed onto the vacuum pumping system with 
the grid uppermost and the crystals on the bottom 
of the cell. The layer was evaporated quickly by 
heating the cell wall with a torch. 

Photosensitization of the evaporated layer only 
developed after exposure to oxygen at high tempera¬ 
tures. The pressure had to be between 20 and 1,000 
p, the temperature between 150 and 350 C, and the 
baking times between 10 minutes and 2 hours. The 
resistance of the unoxidized material is measured in 
thousands of ohms; for the completely oxidized 
material, which turns into a colored insulating layer 
of T1 2 S0 2 , it may be thousands of megohms. For 
good cells, with a few megohms resistance, the aver¬ 
age amount of oxygen taken up in sensitizing is 15 
to 20 per cent of the amount needed for complete 
conversion to T1 2 S02- The X-ray and electron dif¬ 
fraction data of Contract OEMsr-1036 indicate that 
the photosensitive material is a solid solution of 
oxygen in T1 2 S. Sensitive layers are black or dark 
brown with a metallic appearance. 

The effects of contaminating vapors were studied. 
Mercury was harmful to cell properties so that oil 
diffusion pumps had to be used on the vacuum sys¬ 
tems. After much uncertainty and many misleading 
results it was established that water vapor was bene¬ 
ficial in sensitization, apparently playing a catalytic 


role in the oxidation. Absolute ethyl alcohol, hydro¬ 
gen peroxide, and dioxane behave similarly. No sen¬ 
sitization occurs with water vapor only, in the ab¬ 
sence of oxygen. 

Although some elements of the cell preparation are 
very critical, others are not, and quite diverse bak¬ 
ing and sensitization schedules were worked out at 
different laboratories. 

Commercial Production: Special Studies. The de¬ 
velopment of commercial facilities for manufactur¬ 
ing TF cells at GE 4 and at RCA 5 led to variations 
only in minor details from the Northwestern Univer¬ 
sity procedures already outlined. The first object of 
the commercial contracts was to break down the 
process of cell manufacture into small steps capable 
of being adapted to a production flow line and 
handled by technically unskilled operators. The sec¬ 
ond object, emphasized at GE, was to make cells 
which would withstand abuse in the field, such as 
short-time application of overrated voltage, brief 
exposure to sunlight, immersion in hot or cold 
water, temperature cycles, long-time immersion in 
water, exposure to salt spray, and dropping on the 
floor. Navy specifications on resistance of the cells 
to such treatment were worked out by consultation 
with all TF-cell contracts. These requirements led 
to special studies on basing cements, moisture-proof¬ 
ing varnish, and insulating wafers, among other 
things. 

At GE, for controlling quality, a series of syste¬ 
matic tests was set up to be applied to all cells. 
These included: measurement of dark resistance at 
25 C and at 0 C; change of resistance under 0.25- 
footcandle tungsten light; change of resistance 
(polarization) under 22.5 volts in the dark for 8 
hours; S/N ratio on photocell test set; and inspec¬ 
tions throughout each stage of manufacture on such 
points as location, size, and uniformity of grid, 
cleanness of cell, straightness of based cell, etc. 

Several items of special apparatus were designed 
and built at GE. These included cell-cleaning and 
gold-plating baths, grid-ruling machines (seen in 
Figure 12; these are slightly modified from the 
original Northwestern design), exhaust and sensitiz¬ 
ing units, basing devices, and a room with tempera¬ 
ture and lighting regulation which contained the 
inspection and test equipment. 

Over 8,000 type A cells, including rejects, were 
processed at GE before the NDRC contract was 
superseded by a Navy purchase contract. The ini- 







72 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 



Figure 12. TF cell-ruling fixtures on General Electric assembly line. 


tial Navy specification for minimum acceptable 
responsivity had been 38 db S/N ratio for a stand¬ 
ard filtered signal from the test set; the average 
quality of the cells produced was improved enough 
so that it was possible to raise it to 48 db and finally 
53 db before termination of the NDRC contract. 

At RCA, studies w^ere made on glasses and lead- 
ins other than Nonex and tungsten which might be 
suitable for the cells. Studies were made on ruling 
pens and on possible use of decalcomania and silk- 
screen technique for making grid lines. The question 
of mercury versus oil diffusion pumps was examined. 
An empirical formula was found which would rep¬ 
resent the degree of oxidation of the Th>S layer as 
a function of temperature, oxygen pressure, and ex¬ 
posure time. 

Since the RCA contract ran only about six months 
before being terminated as a result of the end of 
the war, only about 300 cells were processed in the 
laboratory (experimental) and about 250 in the 
factory (preproduction). At the end, the factory 
had facilities for production at the rate of 600 cells 
per month. The average S/N ratio (filtered) of the 


laboratory type B cells on the photocell test set 
was about 45 db. 

Testing Procedures 

The simplest test of photoconductive cell sensi¬ 
tivity is the measurement of relative resistance 
change under a standard hololuminous flux. For 
comparisons within a single laboratory, this flux 
needs simply to be reproducible. For interlabora¬ 
tory comparisons, a tungsten lamp at a color temper¬ 
ature of 2848 K is chosen as the source, following the 
system of NIR photometry outlined in the Appendix. 
One-quarter footcandle was used as the standard 
flux in such tests, by tacit agreement. Unfortunately, 
this usually involves high-level nonlinear response; 
it does not indicate the response to modulated radia¬ 
tion nor does it give the dark current noise which 
determines the threshold sensitivity. 

The nonlinearity problem also arises in determin¬ 
ing the relative NIR responsivity of a cell. This is 
determined by measuring the ehT° of the cell for 
some standard source and filter combination, using 
the resultant figure as an indication of long-wave 








PHOTOCONDUCTIVE CELLS 


73 


response. But when nonlinearity is present, a com¬ 
plicated procedure is required, involving equaliza¬ 
tion of response with a source at different distances 
and use of the inverse square law, if an accurate 
value of ehT° is to be obtained. 

Photocell Test Set. To get around these difficulties 
and to make possible accurate interlaboratory com¬ 
parisons of TF cells, University of Michigan Con¬ 
tract NDCrc-185 constructed six photocell test sets 
in 1944. 23 ’ 24 ’ 25 Project Control NS-225 was applied 
to this program. 

These instruments were designed to measure the 
modulated signal and noise output voltages from 
cells with resistances between 0.25 and 40 megohms. 
Since the main military use of the TF cells in 1943 
was in the code communication system described in 
Section 5.2, the set was arranged to duplicate rather 
closely, when desired, the operating conditions of 
this system, while retaining a high degree of flexi¬ 
bility for other experimental adaptations. 

The radiation from the calibrated tungsten source 
at 2848 K color temperature was interrupted me¬ 
chanically by a “light chopper” or sector disk at 90 
cycles (Figure 13). It could be passed, if desired, 
through a 2.4-mm thickness of Corning 2540 filter. 
After a great optical reduction of intensity by a 
known amount, the radiation fell on the cell. This 
was placed in a cathode-follower preamplifier and 
step-attenuator system with suitable indicating 
meters (Figure 14). The tuning of the system to 90 
cycles and the band-pass, which was 1.8 cycles wide 
down 3 db, were regulated by a General Radio Com¬ 
pany’s sound analyzer. 

The optical attenuation of radiant intensity was 



Figure 13. Microflux source box in photocell test set. 



Figure 14. Electrical measuring equipment in photocell test set. 





74 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


produced by forming a small image of the source 
by reflection from a convex surface. By varying the 
source distance and by interchanging a series of 
calibrated aperture stops in the beam, the incident 
flux on the cell could be regulated throughout the 
range of 1 to 100 phlm. By moving a lens, this flux 
could be concentrated on a small spot or spread 
over a large area of the cell. 

The electrical equipment was arranged to meas¬ 
ure cell output voltages between 1 pv and 100 mv. 

With this apparatus it became possible to meas¬ 
ure the responsivity of a cell and its signal equiva¬ 
lent of noise. For a standard input flux, usually 
chosen to be 1 phlm total swing as the light was 
chopped, the output signal could be measured in 
decibels above some reference level. The 0-db level 
was usually taken to be 1 pv rms. By measuring 
noise, then, the S/N ratio and the threshold could 
be determined. The ehT° for a filter could be deter¬ 
mined by the loss of signal in decibels when the 
filter was placed in the path. The flux level was low 
enough so that nonlinearity problems did not arise. 

Various adaptations and modifications were made 
on the test sets by the different contractors using 
them. These included the addition of sets of sectors 
with various numbers of openings for studying fre¬ 
quency response. Noise-frequency data could be ob¬ 
tained merely by varying the tuning of the sound 
analyzer, making proper corrections for bandwidth. 
In some sets, arrangements were made for introduc¬ 
ing other modulated sources than the calibrated 
tungsten lamp or background sources, or for heating 
or cooling the cell, or for moving it around to find 
the most sensitive area. In some experiments the 
output of the amplifier was fed into an oscilloscope 
for study of the wave shape. 

In order to reach the noise level of the cell itself, 
antivibration mountings had to be installed on the 
preamplifier box which contained the cell. This box 
had to be mechanically insulated from the box con¬ 
taining the running motor, the cell had to be placed 
inside a double electrical and optical shield, and 
finally it was necessary to replace all composition 
load resistors by wire-wound resistors because of 
their lower noise. 

The usual reproducibility of interlaboratory com¬ 
parisons of a given cell on different test sets was 
within ±2 db, which is about the reproducibility 
for the same cell after being taken out of a set and 
put back in again. The lack of complete reproduci¬ 


bility is due partly to strange long-period com¬ 
ponents in the TF-cell noise, which make it difficult 
to read the meters to closer than ±2 db, and partly 
to differences in sensitivity over a cell surface as the 
flux distribution or the angle of incidence varies. 

These test sets were furnished to University of 
Michigan Contract NDCrc-185, to Northwestern 
University Contract OEMsr-235, to the General 
Electric Company Contract OEMsr-1322, to the 
Naval Research Laboratory, to the Board of En¬ 
gineers at Fort Belvoir, and to MIT Contract 
OEMsr-1036. After this last contract was termi¬ 
nated, its set was overhauled under Contract 
NDCrc-185 and furnished to RCA Contract 
OEMsr-1486. The electrical measuring portion of 
the test set together with complete blueprints and 
specifications for the microflux source were furnished 
to the British Admiralty Research Laboratory. In 
addition, specifications for the test set were fur¬ 
nished to Farnsworth Contract OEMsr-1094 and 
provided the basis of the equipment constructed by 
that contract for the testing of photomultipliers. 
These specifications for the complete test set were 
also furnished to Wright Field. 

The sets proved to be indispensable in the study 
of variables and maximizing of perfonnance during 
the TF-cell and PbS-cell development, in the stand¬ 
ardizing of interlaboratory comparisons, and in the 
maintenance of quality during production. They 
were and they remain the most valuable and the 
most commonly used instruments for the study of 
the properties of photoconductive and other related 
cells. 

General Properties 

Some general properties of TF cells will be dis¬ 
cussed before going on to the more fundamental and 
theoretical studies. 

Resistance. Cells of different sizes activated in the 
same way, with the same starting material, layer 
thickness, and grid spacing, should have a dark re¬ 
sistance inversely proportional to the grid area. By 
using selected T1 2 S starting material and slightly 
varying the activation schedules, it is possible to 
exert some control over the resistance without seri¬ 
ous loss of responsivity. Low resistance is achieved 
by less oxidation 6 and lower oxidation temperature. 3 
Low resistance cells generally have lower signal re¬ 
sponse, lower noise, and better stability than higher 
resistance cells. 




PHOTOCONDUCTIVE CELLS 


75 


The degree of control of resistance attained in 
production of good cells is shown by the figures on 
various cell properties collected in Table 1. The 
largest and smallest Cashman cells have a ratio of 
areas of about 60, but the dark resistances differ by 
a factor of only 6. The ^x^-inch cell is about as 
small as can be made with good sensitivity if the 
resistance is to be kept under 20 megohms. 

Spectral Response: Filters. The curves of spectral 
response in Figure 7 were taken by Contract 
OEMsr-235 with a Van Cittert double glass mono¬ 
chromator, with the radiant flux measured by a 
calibrated thermocouple. The sensitivity of a ^-inch 
TF cell is such that the current produced by the 
resolved radiation may be measured with a micro¬ 
ammeter. 

The conditions of measurement of the curves in 
most studies made do not guarantee a linear re¬ 
sponse of the cell and associated circuits, so that 
probably the curves are relatively too low where the 
incident intensity is greatest, near 1 p. This non¬ 
linearity does not affect the location of the threshold 
and peak response points but makes it impossible 
to compare measurements taken on different instru¬ 
ments. Contract OEMsr-235 worked out a method 
of correcting for this nonlinearity. 3 ’ 6 Heating effects, 
drift of response with applied voltage and time, and 
possibly hysteresis, are additional sources of error 
in TF-cell spectrometry, which may largely mask 
the true variations from cell to cell and from time 
to time on the same cell. The effect of cell tempera¬ 
ture on spectral response curves is probably small, 
but needs study. Much more careful spectral re¬ 
sponse measurements, under controlled conditions at 
low light levels, such as to guarantee linear response, 
are greatly needed. 

Since the photoconductive effect begins in the 
spectrum between an absorption band at short wave¬ 
lengths and a transmission band at long wave¬ 
lengths, much of the decreasing response of the cell 
at long wavelengths is due to the failure of the layer 
to absorb the radiation. Thus, a TF cell absorbing 
99.9 per cent in the visible may absorb only perhaps 
70 per cent of the incident radiation at 1 p, and 
much less at longer wavelengths. 3 ’ 6 A thicker layer 
absorbs more than a thin one and thus has a sub¬ 
stantially better long-wave response. This is shown 
by the effective holotransmission of the Coming 
2540 filter on the test set; this filter may give a loss 
of signal of 15 db with a visually transparent TF 


cell but only 12 db with an opaque one. However, 
layers too thick do not make good cells as they are 
difficult to sensitize. 

Computed and observed TF-cell ehT values for 
many different filters and sources have been given 
in Chapter 2. The computed values are based on a 
spectral response curve (Figure 7), taken for con¬ 
venience as an arbitrary standard, of a TF cell in 
an intermediate stage of the cell development pro¬ 
gram. 

Lineanty. Early reports on nonlinearity of the 
TF-cell response as a function of incident flux were 
exaggerated and were largely due to poor choice of 
a parameter for measuring response. The funda¬ 
mental photoprocess in the cells is the creation of 
new conduction electrons, and so the proper response 
parameter is the change of cell conductance, or the 
change of current for constant applied voltage, as 
pointed out by MIT Contract OEMsr-1036. 6 
Though plots shovdng change of resistance, or of 
voltage or current, when the cell is in series with a 
load resistor, are important in amplifier discussions, 
they are bound to be nonlinear. Plots of conductance 
also appear to be nonlinear above flux levels gen¬ 
erally accepted as about 10 phlm or about 0.1 pw at 
wavelengths near 1 p, but care must be exercised if 
the internal cell effects are not to be masked by 
heating or amplifier nonlinearity. The curvature in 
Figure 8 may possibly be from the latter cause. 

Though nonlinearity is found at comparatively 
low levels, complete saturation is not reached, and 
there is still some sensitivity left, even in direct 
sunlight. 

Almost all plots of comparative linearity of re¬ 
sponse as a function of wavelength or temperature 6 
or as a function of cell sensitivity 2 > 3 have shown 
that the curvature of the plots depends only on the 
output current and not on the incident intensity. 
This is to be expected regardless of whether the 
nonlinearity is due to the poor choice of response 
parameter, to the external circuit, or to processes in 
the cell itself, and so throws no light on the site of 
the nonlinearity. The equivalence of photocurrent 
behavior for different wavelengths does confirm 
what was already assumed, that all conduction 
electrons are equivalent, regardless of how they are 
initially produced. 

This equivalence even appears to extend to the 
PbS cell. This cell has a greatly extended region of 
linearity in plots of output versus incident flux, as 



76 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


compared to the TF cell. Probably this extra linear¬ 
ity, like the good frequency response of the PbS cell, 
is simply the result of its poor sensitivity; for much 
smaller currents are produced in it than are pro¬ 
duced by the same light level in a TF cell. 

Variation of Signal and Noise with Cell Area. 
An elementary consideration 30a of the output signal 
and noise voltage from two identical photoconduc- 
tive cells placed in parallel shows that, because of 
their mutual shunting effects, the signal must be 
half that from one cell alone, and the noise less by 
V 2. On generalizing to many elements in parallel, 
evidently the signal varies inversely with the area, 
the noise inversely with the square root of the area. 
The S/N ratio thus varies inversely with the square 
root of the area, just as for photoemissive cells. 
This holds true regardless of whether the total flux 
falls on only one cell or is divided between the cells 
(up to illumination levels where saturation effects 
begin to appear), since the signal emfs developed 
in all cells are in phase. 

The data shown in Table 1 for various sizes of 
Cashman cells confirm these predictions within ±3 
db, which is fair agreement considering the varia¬ 
tion in layer thickness, sensitization, resistance, and 
grid spacings. No theory which includes these fur¬ 
ther variables has as yet been presented. The type B 
cells show an unexpectedly high signal response for 
their large area. 

Threshold Sensitivity. The threshold sensitivity 
may be indicated by the hololuminous signal equiva¬ 
lent of noise. For type A cells, as shown in Table 1, 
this quantity is of the order of 10" 9 him (total 
swing; square wave, chopped) for the tungsten 
source with a 2540 filter as in the test set. This cor¬ 
responds to a threshold sensitivity of the order of 
10" 11 watts (total swing; square wave, chopped) for 
radiation of wavelengths near 1 p. Both values de¬ 
pend, of course, on the noise, determined by the 
bandwidth, which is 1.8 cycles in these measure¬ 
ments. In the voice communication system described 
in Section 4.4.2, the rms variation of signal which 
was the equivalent of noise in a type B TF cell was 
measured under Contract OEMsr-990 to be about 
3X10~ 8 effective him, the incident signal being from 
a cesium-vapor source modulated at 1,500 cycles, 
and the bandwidth being about 600 cycles. The 30- 
fold difference from the test set threshold values is 
due principally to the large bandwidth. 

Heat Detection. The possible sensitivity of the 


TF cell as detector of warm military targets was 
investigated by Contract OEMsr-235, with the 
results shown in Figure 15. For a target 200 C 
above background, the TF cell is less sensitive by a 
factor of 10 3 than an uncooled PbS cell, and by 10 3 
than a dry-ice-cooled PbS cell. Because of the 



Figure 15. Sensitivity of TF and PbS cells at room 
temperature as thermal detectors. 


shorter long-wave threshold of the TF cell, the slope 
of its response as a function of target temperature 
is much steeper than that of the PbS cell, so that the 
TF cell is relatively much worse for targets closer 
to the background temperature and becomes com¬ 
parable to the lead sulfide cell only at target tem¬ 
peratures of 500 C or over. 

Secondary Emission. A special cell was con¬ 
structed under Contract OEMsr-235 containing an 
electron gun and electrodes for measuring the sec¬ 
ondary electron emission of a T1 2 S layer oxidized 
and unoxidized. This is important because of the 
possible use of such layers in infrared iconoscopes 
or other image-forming devices. The voltage of the 


..—tT^tb 1 iih i Bnrmf * i T ' 



































































PHOTOCOINDUCTIVE CELLS 


77 


incident electrons was varied from 200 to 750 volts. 
Oxidation and subsequent activation do not produce 
a marked change in the yield of secondaries. The 
most favorable yield of 1.35 secondaries per primary 
electron was obtained over a broad region centered 
at about 450 volts primary accelerating potential. 
This yield is too low by a large factor for further 
consideration in image devices. It would be interest¬ 
ing to determine whether a low secondary yield is 
characteristic of all sulfides. 

Photovoltaic Effects. In the studies carried out in 
1941 it was found possible to make very sensitive, 
but not very stable, low-resistance photoemf thal- 
lous sulfide cells by proper oxidation-activation. 
High-resistance cells showing photovoltaic effects 
occasionally appeared during the study of photo- 
conductive variables, but they are very rare among 
the cells made by the final manufacturing processes. 

Under Contract OEMsr-235 it was found that the 
photoemf effects appeared to be associated with 
large gradients of oxygen concentration in the T1 2 S 
layer which very likely were altered by oxygen 
migration, with cell age or under voltage or sunlight, 
to produce the instability. Such unstable gradients 
were very undesirable and appeared to be unneces¬ 
sary for creating photoconductivity; indeed, it was 
thought that the great improvement in the TF cells 
introduced by preoxidizing the T1 2 S was due to the 
increase in uniformity of distribution of the oxygen 
throughout the finished cell. 

Now that better amplifiers for low-resistance de¬ 
vices are available, and more experience in handling 
T1 2 S has been accumulated, it might be valuable to 
study the photoemf effects further. 

Fundamental and Theoretical Studies 

Study of Properties and Interrelations. Extensive 
and varied studies have been made by all contracts 
on TF-cell signal and noise and their dependence on 
some or all of the following factors: cell design; 
TloS preparation, evaporation, and sensitizing proc¬ 
esses; grid spacing and area, cell resistance, and 
layer thickness; crystal structure and orientation; 
cell age, and pre-exposure to sunlight and voltage; 
load resistors and circuit design; signal flux, wave¬ 
length, and size of illuminated area; voltage,, fre¬ 
quency, bandwidth, steady background light, and 
cell temperature; and on the interrelations of all 
these factors. 

The factors are very difficult to separate for indi¬ 


vidual study. Early analysis was qualitative and 
semi-intuitive, and was aimed primarily at produc¬ 
tion of better cells by any means, whether fully 
understood or not. Later, the study of many factors 
was perforce statistical, relying on large numbers of 
cells to average out the obvious batch, process, and 
operator variations. Even for studies which can be 
made on a single cell, as of variations in the incident 
flux or in external conditions, few exhaustive studies 
of all properties were made on the same cell even 
though results are known to differ very much from 
cell to cell. Many variables have proved to be in¬ 
terdependent in unexpected ways, so that early 
measurements have been repeatedly invalidated by 
later results. The pressure for production has of 
course played its part in slowing down these studies 
and in making many of them hasty and inconclu¬ 
sive. 

As a result, confirmed quantitative relationships 
are few. They include the relation of signal output 
to frequency, which was established early; 2 cell 
resistance as a function of temperature; 3>6 ’ 9 and the 
relationships involved in the steady light saturation 
effects and in the decay response patterns. 6 ’ 9 The 
variation of noise with frequency was cleared up in 
1944 by Contracts NDCrc-185, OEMsr-235, and 
OEMsr-990. 3 ’ 30 The first reliable measurements of 
output signal linearity as a function of flux level 
(modulated) over great ranges of intensity were not 
made until 1945 3 and may still be doubtful at the 
higher ranges because of possible amplifier over¬ 
loading. Phase lag needs more study, both theoreti¬ 
cal and experimental; 9 likewise the relations be¬ 
tween time constant, dark resistance, and degree of 
oxidation of the T1 2 S layer. 6 

Navy Contract NObs-25392, which is continuing 
the work of Contract OEMsr-235 at Northwestern 
University, is apparently providing opportunities 
for studying background effects under carefully con¬ 
trolled conditions for the first time. As usual with 
TF cells, this work throws doubt on many earlier 
results; in this case because background effects are 
found at very low light levels, possibly comparable 
with the “darkness” in which earlier experiments 
were thought to have been conducted. 

Theoretical interpretation is in about the same 
shape. It seems to be agreed, as a result of the MIT 
work under Contract OEMsr-1036 measurement of 
thermoelectric effect in TF cells, that the photoeffect 
is due to “defect” conduction. In this kind of con- 






78 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


duction the light quantum ejects an electron from a 
filled conduction band up to a bound or “trapped” 
state (Figure 16); the conductivity is produced by 
the mobility of the “hole” left behind in the conduc¬ 
tion band. 



Figure 16 . Energy state diagram for defect con¬ 
ductor. 


Theoretical analysis has been given of the noise- 
frequency relationship, 28 * 30b but the physical mech¬ 
anism responsible for the relationship is in dis¬ 
pute. Simple interpretations are possible for steady 
response and time-lag effects. Apparent weak res¬ 
onance effects at certain frequencies when some 
background light is present have not been ex¬ 
plained. 3 ’ 10 Altogether the NDRC literature on TF 
cells is a mine of exciting material for discussion 
and a stimulus to further experiment. It should be 
consulted in the original by the reader interested in 
the problems of semiconductors and photoconduc¬ 
tors. 

Assumed Mechanism. The very general results 
which can be given here will perhaps be better un¬ 
derstood and the purpose of some special studies 
will be clearer if a simple picture of the photocon- 
ductive process is outlined first. 

A band of low-energy states (Figure 16) in which 
electrons are free to move throughout the T1 2 S 
crystal lattice is conceived to be filled completely 
with electrons at absolute zero temperature, so that 
no net motion of charge can take place under an 
applied field and the conductivity is zero. At some 
distance in energy above the top of this band is a 
group of trapped electron energy states, probably, 
according to those working under Contract OEMsr- 
1036, localized at the oxygen atoms. Into these 
states electrons may be thermally or optically ex¬ 


cited, leaving behind a conduction hole which, under 
the influence of an external electric field, is free to 
move throughout the lattice by means of the retro¬ 
grade motion of the electrons surrounding it. Even¬ 
tually the holes may be neutralized by “collision” 
with a trapped electron. 

The number of these holes per unit volume deter¬ 
mines the resistivity of the material. When no 
radiation is falling on the cell, this number is an 
exponential function of temperature. When radia¬ 
tion impinges, it increases the conduction if it has 
an energy per quantum greater than the threshold, 
hv 0 or hc/\ o, w T hich is the minimum difference of 
energy between the lower and upper states (h is 
Planck’s constant, c is the velocity of light, v 0 and 
X 0 are the threshold frequency and wavelength, re¬ 
spectively). This energy threshold therefore deter¬ 
mines both the long-wavelength threshold and the 
behavior of the resistance with temperature. 

This picture of defect conduction was not adopted 
until after the crucial thermoelectric experiment in 
1945. Previously the conduction had been assumed 
to be of the opposite or “excess” type as described 
in Mott and Gurney, 33 and the comparison of view¬ 
points is on this account somewhat confusing. In 
excess conduction, the trapped electrons are assumed 
to be below, and to be raised to a higher, empty 
conduction band by radiation or heat; the electrons 
themselves therefore carry the current. 

Actually it is now established that both kinds of 
conduction commonly take place simultaneously 
(intrinsic conduction) in any real semiconductor, 
and the effect observed externally depends on their 
relative proportions under the conditions of the 
experiment. The early reports based on the excess 
theory can apparently be converted to the defect 
theory by a simple change of notation if the con¬ 
duction is confirmed to be of the latter type. 

Variation of Resistance with Temperature. As ex¬ 
pected for a semiconductor, the resistance varies 
strongly with the temperature T, the coefficient 
being about 6 per cent per degree centigrade near 
room temperature. Theoretically 32 ’ 33 the relation 
should be 

R = Ae u / kT . 

The temperature dependence of the coefficient A is 
different in different theories but is small and may 
be neglected. The quantity U is called the “thermal 
activation energy” and should be equal to hv 0 /2 in 


■RlSISJGFEfr 









PHOTOCONDUCTIVE CELLS 


79 


the simple theory just outlined; k is the Boltzmann 
constant. An experimental plot of log R versus l/T 
is shown in Figure 11 3 and obeys this equation very 
well. The activation energy determined from the 
slope of the line for the cell shown is 0.47 electron 
volts. On one good British cell U was found to be 
0.43 ev. 9 Under Contract OEMsr-1036 the activa¬ 
tion energies of many cells were measured by this 
method, seeking a relation with the degree of oxida¬ 
tion of the layers, but no relation was apparent, 
though the change of resistance (the constant A) 
with oxidation was very marked. 6 Values of U 
varied from 0.41 to 0.77 ev, but it is not clear what 
range of values are especially characteristic of 
“good” cells. 

The values of U of 0.43 and 0.47 ev correspond, 
on the simple theory, to optical thresholds of 1.44 
and 1.31 p, agreeing well enough with the photo- 
conductive thresholds commonly observed and with 
the onset of optical absorption, as far as is known. 
Obviously, thermal, photoconductive, and absorp¬ 
tion studies on the same cell are greatly needed. 

Response to Steady Flux. The response of TF 
cells as a function of the total steady flux incident 
has been measured by all contracts. Large differ¬ 
ences are present in the absolute values of response 
found by different contracts, but these are probably 
due simply to variations in sensitivity of the cells 
tested. The general shape of the curves is the same 
in all cases. The relative change of resistance, 
A R/R d , has been fitted by those working under 
Contract NDCrc-185, on the basis of the theory of 
excess semiconduction, by an equation of the form 

A R _ V 1 + y'P - 1 

Rd ~ V 1 4 -y'P ~ C’ 

where y' and C are constants of the material and P 
is the radiant power falling on the cell. 9 

This theory was adapted to the point of view of 
defect or intrinsic conduction by Contract OEMsr- 
1036, using a slightly different notation. There re¬ 
sulted an equation of essentially identical form 
which fits the data equally well. 6 

Growth and Decay of Response to a Single Pulse. 
To throw light on the origin and nature of the time 
constant observed in the frequency response curves, 
studies were made under Contract NDCrc-185 with 
an oscillograph of the growth of response when a 
steady light is suddenly turned on and of the re¬ 
sponse to a flash of 10 to 50 psec duration. 9 Other 


work by Contract OEMsr-1036 showed the decay 
of response after a microsecond pulse from a flash 
lamp. 6 

The simple theory formulated by those working 
under Contract NDCrc-185 predicted rates of 
growth of response somewhat different from those 
observed. It was suggested that the discrepancies 
were due to “space charge” (polarization) inside 
the crystal or to the shunting effects of the unillumi¬ 
nated portion of the cell. The predicted effect of 
pulse length on initial slope of decay was not con¬ 
firmed. Only the behavior of the decay tail could be 
taken to agree with theory. 

Those working under Contract OEMsr-1036 re¬ 
peated the studies of response decay with many 
cells as a function of voltage, light intensity, wave¬ 
length, and temperature. They adapted the calcu¬ 
lations to their theory of T1 2 S as a defect conductor. 
Following the microsecond pulse, the response builds 
up to a peak in about 5 X 10~ 5 second and then 
decays. To explain the initial shape of the decay 
curves, they postulated a distribution of shorter 
time constants in the material in addition to the 
dominant time constant t characterizing the decay 
tail. The value of t was found to vary with tempera¬ 
ture as exp (U'/kT). The quantity U ' was a constant 
for a given cell and was determined to be 0.65 ev 
on a cell whose activation energy U was 0.54 ev. 
The difference between U and U f might be explained 
by a variation of conduction hole velocity or of cap¬ 
ture cross section with temperature. 6 

The value of the time constant t at room tempera¬ 
ture, for a group of cells of the same size, was found 
to increase with decreasing cell resistance, from 
5 X10" 4 second for a 450-megohm cell to 3 X 10~ 2 
second for a 1.6-megohm cell. It appears that the 
law that t is proportional to 1/R is expected from 
either theory. Inspection of the data given under 
Contract OEMsr-1036 on 11 very different cells, 
scattered throughout the resistance range just given, 
shows that this law is followed to ±40 per cent. This 
is surprisingly good agreement, considering the other 
sources of variation which are neglected in deriving 
the law. The value of t seems not to depend on the 
amount of oxidation, though R does. Perhaps the 
oxidation accounts for most of the deviations of R 
from the 1 /t law. 

As was expected, t is found to be independent of 
the wavelength of the incident light. 

Signal and Noise: Frequency Response. The con- 







80 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


duction electrons must be thought of as continually 
boiling up from the conduction band and condens¬ 
ing again out of the upper states. The rate of boiling 
up is constant at constant temperature in the dark. 
The rate of condensation, which is probably due to 
neutralization of holes, depends on the number of 
electrons in the excited state. If an excess boiling 
rate occurs for an instant, whether from a thermal 
fluctuation (noise) or because of incident flux (sig¬ 
nal) , a finite time is required to increase the number 
of conduction electrons so that equilibrium is re¬ 
stored; if the boiling rate is lowered, again there is 
a time lag for the conduction to decay. 

Because of the lag, whether explainable by this 
mechanism or by some other, both signal and noise 
are strongest at low frequencies. 

This kind of excitation noise is in addition to the 
thermal or Johnson noise caused by fluctuations in 
the number of conduction electrons or holes reaching 
the electrodes at any instant. 30b The latter is present 
in every conductor. It is a function of resistance 
only and is independent of voltage or frequency for 
constant bandwidth (white spectrum). The relation 
between the excitation noise and the Johnson noise 
is similar to the relation in a triode amplifying tube 
between noise introduced by the grid and “shot 
noise.” The excitation noise, just like the excitation 
signal, is amplified by the conduction process in 
proportion to the drift velocity of the holes, which 
is proportional to the applied voltage. Because of its 
time constant, this noise has no high-frequency com¬ 
ponents and decreases with increase of frequency 
up to about 3,000 cycles, above which only the 
Johnson noise remains. This combined noise spec¬ 
trum closely resembles contact noise, which has been 
investigated for carbon microphones and thin metal¬ 
lic films. 

The linearity of signal and noise with voltage is 
observed experimentally, except in occasional un¬ 
explained cases, up to voltages of 100 to 200 volts 
where some kind of internal cell breakdown appears 
to take place, accompanied by large increases in 
noise. Cells with wide grid spacing, like the type B 
cell (15 lines per inch), stand voltage better than 
cells with narrow spacing, as would be expected. 

The theoretical expressions for rms signal and 
noise, as functions of frequency /, when a time con¬ 
stant t is involved, both take the form 

Cx 

VT+l2 WT 2 ’ 


where C includes the quantum efficiency, electron 
mobility, voltage, or other parameters. 

Since the d-c response (/=:0), represented by 
the numerator, is proportional to the time t during 
which conduction electrons can accumulate, it is 
favored by a long time lag. But with a long lag, 
the denominator grows rapidly with frequency, and 
the frequency response is poor. It is observed ex¬ 
perimentally that the cells most sensitive to d-c 
light usually have poor frequency response. The 
better frequency response of PbS cells as compared 
with TF cells may be directly connected with their 
lower sensitivity. 

Those working under Contract OEMsr-1036 
found that the absolute signal response of any cell 
measured as a function of frequency on the test set 
could be predicated by the above expression within 
2 db up to 700 cycles, simply from measurements 
on the d-c response and on the decay curve time 
constant. 

Fluctuations are equalized more quickly—the 
time constant is shorter—when there are more elec¬ 
trons in the conduction band. This occurs at higher 
temperatures, or with increased light intensity on 
the cell. Both conditions are observed experimentally 
to decrease the sensitivity and to improve the fre¬ 
quency response. However, reliable quantitative 
measurements are few and are apparently not cov¬ 
ered by any simple theory. 

Experimentally, the sensitivity of TF cells falls 
at low temperatures as well as at high, even though 
the time constants increase with lower temperature 
as expected. This must mean that the constant C 
varies with temperature. In some other photocon¬ 
ductors, such a variation has been attributed to 
changes in quantum efficiency. 32 The possibility of 
compensating for the poor frequency response at 
low temperatures, in an aircraft communication 
problem (Section 4.4.4), by adding background light 
was considered under Contract OEMsr-990; this 
proposal deserves testing. Extensive further tem¬ 
perature, background, frequency, and related studies 
would be very valuable. 

Returning to the frequency variation of signal 
and noise without background at room temperature, 
it is evident from the expression above that at large 
frequencies where fx > 1 both signal and noise (for 
constant bandwidth) should fall off as 1//—or at 
6 db per octave—above about 100 cycles. This is 
observed experimentally for TF cells, as seen in 
Figure 9, except for occasional unexplained cases in 







PHOTOCONDUCTIVE CELLS 


81 


which the rate is as low as 4 db or as high as 8 db 
per octave. 

Since signal and excitation noise both vary in the 
same way, the S/N ratio should stay constant up to 
frequencies where Johnson noise begins to be sig¬ 
nificant. This also is confirmed by Figure 9 up to 
about 1,500 cycles, with perhaps as good accuracy 
as circuit and bandwidth corrections will permit. 

Phase Lag. The existence of a time lag in the 
response of a cell, which leads to the frequency 
response behavior just discussed, must also lead to 
phase lags between sinusoidally modulated incident 
light and the output response. The expression for the 
lag should be tan -1 2jt/t. The values measured under 
Contract NDCrc-185 over the range 30 to 800 cycles 
agree only roughly with this expression. 9 The 
amount of lag is about right in the middle of the 
range, the values decrease with background light 
as t decreases, but the tangent of the angle is not 
proportional to frequency as it should be. One ex¬ 
planation for the discrepancy might be experimental 
difficulties; another might be the space charge and 
polarization effects not considered in the theory. 

Thermoelectric Effect. A special TF cell was con¬ 
structed under Contract OEMsr-1036, the layer 
being formed between single metal electrodes with¬ 
out gridwork, with arrangements for heating one 
electrode and cooling the other by external baths. 
The direction of the thermoelectric effect was meas¬ 
ured. This direction determines whether excess or 
defect conduction is present. Since the charge car¬ 
riers will diffuse from the hot junction to the cold 
junction, the latter becomes charged negatively in 
excess conduction, positively in defect conduction. 

Under Contract OEMsr-1036 it was found that a 
pure T1 2 S layer had excess conduction as in the 
picture outlined above; but that the sign reversed 
on oxidation, and the sensitized layer had defect 
conduction. A layer made from preoxidized material 
showed defect conduction at all times. Earlier Rus¬ 
sian work showed low-resistance T1 2 S to have excess 
conduction, high-resistance to have defect conduc¬ 
tion, but the change of resistance produced by oxida¬ 
tion (in air) was opposite to that observed in all 
these studies, so it is not positive that the same 
chemical substance is involved. 

The direction of the thermoelectric effect would 
earlier have been regarded as conclusive proof of 
the invalidity of the excess conduction theory. But 
some recent work on other semiconductors indicates 
that the direction of the Hall effect (transverse 


voltage developed when a current flows through the 
material in a magnetic field), which is the only 
other way to determine the sign on the charge car¬ 
riers, may give exactly opposite results from the 
thermoelectric results at a given temperature; and 
that a study of both effects over a large tempera¬ 
ture range is sometimes needed in order to be certain 
of the interpretations. Such a study would evidently 
be valuable here. 

Crystal Structure. It was expected that some of 
the photoconductive effects observed, particularly 
the process of sensitizing with oxygen, would be 
associated with changes in crystal structure or orien¬ 
tation. Changes of this kind had appeared to be very 
important in the production of photovoltaic cells 
in the early work under Contract NDCrc-185 in 
1941. 

Workers under Contract OEMsr-1036 therefore 
undertook X-ray and electron diffraction studies on 
the T1 2 S in various stages. 6 X-ray powder pictures 
were taken of the fused material and of material 
taken from photocells after various amounts of 
oxidation. For electron diffraction, photosensitive 
surfaces were prepared in the diffraction apparatus. 
Transmission patterns for pure T1 2 S were made 
with the layer deposited on 0.01-p thick Formvar 
films. Grazing incidence measurements were made 
on layers evaporated on Aquadag coated glass (to 
prevent charging up of the surface), the layers ex¬ 
tending over an adjacent normal cell gridwork for 
checking the photoconductive properties. 

The pure T1 2 S shows the structure reported in the 
literature. The structure may be thought of as 
lamellar, the fundamental unit being a triple of 
Tl-S-Tl layers. Conduction is thought to take place 
most easily in the T1 planes. In all evaporated lay¬ 
ers, these planes were found to be parallel to the 
backing surface. 

In all photosensitive cells, the T1 2 S phase is pres¬ 
ent; in some cases, a second, yellow, phase is also 
found. Its structure differs from that of any pre¬ 
viously known thallium compounds, but it has been 
tentatively identified as T1 2 S0 2 . 

During photosensitization of the T1 2 S layer, the 
original orientation persists and the crystallites 
grow in size. However, the diffraction pattern be¬ 
comes more diffuse and indicates that the structure 
has probably been disrupted by solid solution of 
some oxygen. With more complete oxidation only 
the T1 2 S0 2 patterns appear. 

It is supposed that the oxygen first fills in the 


te g£r> 





82 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


interstices between the thallium planes of adjacent 
triple layers. Complete filling of these holes would 
require 0.5 mole of 0 2 per mole of T1 2 S; photosen- 
sitivity is produced with 0.1 to 0.3 moles 0 2 . 

Theories of the Photoconductive Process. From 
the various experiments on different cells, those 
working under Contract NDCrc-185 were able to 
compute the orders of magnitude of the fundamental 
constants entering into their original excess conduc¬ 
tion theory. They assumed that the volume of the 
TF cell layers is about 10“ 5 cubic centimeter and 
that the quantum efficiency or yield is about 0.1 
electron excited per quantum absorbed. The data 
then lead to a value of the order of 10 17 for the 
total number of impurity centers per cubic centi¬ 
meter from which the electrons are excited. Of these, 
10 14 are unoccupied at absolute zero. The number 
of conduction electrons present in the dark at room 
temperature is about 0.5 X 10 14 . The mobility of the 
conduction electrons comes out to be 2 centimeters 
per second per volt per centimeter. The product of 
electron velocity times capture cross section is about 
10" 14 cubic centimeter per second, which is small 
compared to values of this product for other semi¬ 
conductors. 

Those working under Contract OEMsr-1036 con¬ 
cluded that the phenomena would be best explained 
by defect conduction, with the holes and the elec¬ 
trons in the conduction band migrating in opposite 
directions under the influence of an applied field, the 
two having different mobilities. The electrons are 
much more mobile and cause excess conduction to 
dominate in pure T1 2 S. In the more sensitive oxidized 
layer of an active TF cell, the oxygen atoms added 
during sensitization trap these electrons, leaving the 
holes to act as the dominant charge carriers. 

The action of radiation is considered to be as fol¬ 
lows: An electron is raised to an excited state, still 
bound to its parent atom. In the absence of oxygen, 
it usually falls back without leaving the atom but 
not always. Even in pure T1 2 S it will sometimes 
leave the parent atom and so cause some photosen¬ 
sitivity. This effect, if it is not spurious, is weaker 
by a factor of over 10 4 than the photoeffect in good 
cells. 6 

After a cell is oxidized, the picture is different. 
An excited electron is then likely to jump to a 
nearby oxygen atom, creating an O" ion and leaving 
the positive hole behind which drifts slowly to the 
cathode. According to this theory, the presence of 


the hole permits many “secondary” electrons to 
pass through the material to the anode without 
being limited by space charge. The number of O” 
ions slowly decays as they are neutralized by holes, 
or, more literally, as the electron on such an ion 
jumps to a neighboring positive ion which has just 
been created. 

From this theory and the experimental data re¬ 
corded under Contract OEMsr-1036, the room tem¬ 
perature electron mobility is found to be about 60 
centimeters per second per volt per centimeter. As¬ 
suming a “primary” quantum yield of about 30 per 
cent, the hole mobility comes out about 1 centimeter 
per second per volt per centimeter. The equilibrium 
density of electrons determined from flash decay 
experiments is 2 X 10 13 per cubic centimeter, from 
which the activation energy can be independently 
computed to be 0.51 ev, agreeing with the measured 
thermal activation energy. The recombination prob¬ 
ability (corresponding to velocity times cross sec¬ 
tion in the excess conduction theory) is 2 X 10" 12 
cubic centimeter per second; about 40 per cent of 
the recombination centers decay up to ten times 
faster than indicated by this figure. From the re¬ 
combination probability and from estimates of the 
hole velocity and of the number of holes which 
collide with oxygen ions without recombining, an 
oxygen ion cross section of radius 2.5 A is indicated, 
agreeing well enough with known interatomic dis¬ 
tances in the lattice. 

This theory has not been applied to the noise- 
frequency problem, but no doubt the Johnson noise 
would be regarded as mainly electronic, with the 
excitation noise behavior accounted for by the decay 
time of the O" centers. 

One advantage of this theory, in addition to its 
explanation of the thermoelectric results, is that it 
accounts for the enormous amount of oxygen needed 
to produce photosensitivity—at least 0.1 mole per 
mole T1 2 S, or about 10 22 atoms per cubic centi¬ 
meter. The function of so much oxygen is not clear 
by the excess conduction theory, which demands a 
density of impurity centers only 10 -5 as great. 

The defect theory also will account for the appar¬ 
ent failure of the oxygen to affect the activation 
energy or the photothreshold or the absorption band 
of the TF cell layer, which is otherwise hard to 
understand with these large concentrations of oxy¬ 
gen. 

On the other hand, those working under Contract 






PHOTOCONDUCTIVE CELLS 


83 


OEMsr-235 point out that the long-wavelength 
threshold and the location of the TF-cell absorption 
bands have changed from their wavelengths in the 
early Case cell, and that this is difficult to explain if 
the photosensitive material consists only of a solid 
solution of oxygen in T1 2 S. This group suggests that 
more careful measurements should be made of the 
spectral absorption of layers during oxidation, es¬ 
pecially at wavelengths longer than 1 p, in the search 
for the appearance of new bands possibly arising 
from new compounds in the layer. 

Present Status 

The work begun under Contract OEMsr-235 is 
being continued at Northwestern University under 
Navy Contract NObs-25392. 

Recommendations 

No further development work appears to be neces¬ 
sary on the TF cell proper as a military detector of 
NIR radiation, but the further study of the funda¬ 
mental properties of such layers, the comparison 
with other materials, and the working out of more 
satisfactory theories of the photoconductive process, 
are all matters of the greatest importance, both 
scientific and military. 

3 3 2 Lead Sulfide Cells 

Initial State of the Art 

The rectifying properties of galena have been 
known since 1874. Case, in 1917, seems to have 
been the first to detect its photoconductive proper¬ 
ties, 10 and Lange in 1931 reported its photothreshold 
as 4.5 p. The photoproperties of natural PbS crys¬ 
tals vary greatly from sample to sample. 

Cashman, in his 1941 semiconductor studies, de¬ 
scribed in Section 3.3.1, found that PbS could be 
evaporated in vacuum without decomposition. In 
1944, when the NDRC TF-cell development was 
nearly complete, attention was again turned to 
Ag 2 S, MoS 2 , and PbS as possible photodetectors for 
longer wavelengths, with the results which will now 
be reported. 

This NDRC work was thought at the time to be 
the first production of photoconductivity in syn¬ 
thetic preparations of PbS, but it was discovered 
shortly that the Germans had been using PbS cells 
as HR and NIR detectors in military equipment 
for several years. 


German Cells. Some of the properties of these 
cells have been described in a report on the German 
(Zeiss) Lichtsprecher 250/130 NIR and IIR nar¬ 
row-beam voice communication system (Section 
4.3.1). 14 A number of captured German cells were 
studied by the University of Michigan under Con¬ 
tract NDCrc-185, by Northwestern University 
under Contracts OEMsr-235 and OEMsr-990, and 
by Harvard University under Contract OEMsr-60, 
as well as by the Naval Research Laboratory. 12 * 13 

These cells were of two general types. The first, 
or “button,” type consists of a sensitive layer from 
y 2 millimeter square (used in the Lichtsprechers) to 
6 millimeters square, without grids. The layer is on 
a glass backing plate contained in a cylindrical 
plastic case about 1 inch in diameter and % inch 
thick, with a small opening in the front, just over 
the sensitive surface. This opening was covered 
with a thin glass or quartz window. The cells were 
made in three series, with plain numbers, numbers 
prefaced by G, and numbers prefaced by GG. These 
appear to represent successive levels of excellence 
attained in a manufacturing program extending over 
several years. 

After capture of the Zeiss works, the process of 
manufacture of the layers for the small button 
cells was described. 13 They were made in air, many 
at one time, by coating a long glass strip, about % 
inch wide, with evaporated metal contacts extend¬ 
ing the length of each edge. A uniform layer of PbS 
was deposited along the whole length of the strip. 
In one process this was done chemically and in an¬ 
other by evaporation. The strip may then have been 
subjected to further unknown sensitizing operations. 
Then two more metal strips were evaporated 
along the edges to improve the contacts. The strip 
was broken into segments, y 16 to % inch wide, each 
of which formed a complete cell. Contacts were 
made inside the plastic box by phosphor bronze 
clips. It was asserted that only one man had the 
skill required to turn out these cells. Apparently 
many hundreds of the cells were made during the 
war. 

In the second type of German cell, the Elac 
(Electroakoustische Gesellschaft), the PbS layer 
was in the outer bulb of a kind of Dewar flask (as 
in Figure 17), evaporated over the end of the re¬ 
entrant tube in the flask. The re-entrant tube was 
designed to hold chunks of solid C0 2 , pressed 
against its end by a plunger to keep the layer cold. 




84 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


The improved response of the PbS cell at low tem¬ 
peratures was also discovered independently under 
NDRC. These Elac cells were made with sensitive 
areas up to 1 inch square. The cooled cells were 
apparently used during the war as heat detectors 
against ship targets in the English Channel. Work 
had also been done on their use as the sensitive 
elements in proximity fuzes. A few cells of some¬ 
what similar type but with inferior characteristics 
were apparently produced also by Allgemeine Elek- 
trische Gesellschaft. 



l 




Figure 17. Diagram of re-entrant PbS cell. 

The fundamental properties of the German cells 
seem to be similar to those of the NDRC PbS cells, 
but the recent samples of the latter appear to be 
more stable and much more sensitive. The long- 
wavelength thresholds of both appear to lie at about 
3.6 p, and a factor of 10 to 100 in sensitivity is 
gained in both by going to dry-ice temperature. 12 

British Cells. Before the end of the war a few 


British PbS cells had been made, following NDRC 
and German methods and designs. A few of these 
also came to this country for testing. 

No other foreign PbS-cell manufacture is at pres¬ 
ent known. 

Course of Development 

History of Development. The PbS-cell program 
in this country during the war developed late and 
was sponsored only by NDRC. Although various 
branches of the Services showed great interest in the 
work, no formal Service project requests had been 
set up before the end of the war. 

Because the discovery of the cell is so recent, its 
great promise as an NIR and HR detector has not 
yet been fully explored. 

The PbS cell was developed by Northwestern 
University under Contract OEMsr-235. Following 
the successful development of the TF cell, atten¬ 
tion had been turned in February 1944 to possible 
selective photodetectors of longer wavelengths. 
Crystals and evaporated layers of several materials 
were studied without much success until the fall of 
1944, when it was found that oxygen photosensi¬ 
tizes lead sulfide. The sensitivity of the first PbS 
cells made was very low, but by December 1944 it 
had been brought up enough for accurate measure¬ 
ments of response characteristics. At that time, two 
captured German PbS cells were received and 
studied. 

In 1945, further increases of sensitivity were ob¬ 
tained, until the uncooled PbS cells finally had S/N 
ratios on the test set only 10 to 20 db less than the 
ratios for TF cells of the same area. With the lead 
sulfide, moreover, cells could be made in sizes down 
to less than 1 square millimeter. These consequently 
had S/N ratios about equal to the ratios of the 
smallest good TF cells (about % inch square) when 
measured on the test set with a filter. 

The PbS cells have a remarkable combination of 
characteristics: photoresponse to long wavelengths, 
good frequency response, and a very high sensitivity 
compared to thermal detectors such as bolometers. 
This combination made possible the use of a new 
region, the IIR, both for purposes of voice com¬ 
munication and for purposes of heat detection. 

To explore this region, atmospheric transmission 
measurements in the IIR were undertaken in 1945 
by Harvard University Contract QEMsr-60 (Sec¬ 
tion 4.8 and Chapter 9). Studies preliminary to the 























PHOTOCONDUCTIVE CELLS 


85 


development of HR voice communication systems 
were begun by Northwestern University Contract 
OEMsr-990 (Section 4.8). A device using PbS cells 
for the detection of heat radiation from military tar¬ 
gets such as men and ships was built and tested by 
University of Michigan Contract NDCrc-185 
(Chapter 9). 11 

Military Importance: The Intermediate Infrared. 
The intermediate infrared [IIR], in which the PbS 
cell is the first photosensitive detector, is taken to 
extend from the NIR limit at 1.4 p, set by the TF-cell 
threshold to the great atmospheric water-vapor ab¬ 
sorption band at 6 p. There is promise that other 
sensitive photodetectors may shortly become avail¬ 
able which will have thresholds much closer to the 
6-p limit than does the present PbS cell. 

The IIR has two advantages over the NIR as a 
channel for voice and code communication and rec¬ 
ognition systems. The first lies in the compara¬ 
tively great security of* IIR sources at present, 
since, when properly filtered, they can be detected 
only by the PbS or some other long-wavelength 
cell. Such sources can be made almost impossible 
to detect by NIR-sensitive devices, which are now 
and will undoubtedly remain for many years many 
times more numerous and more varied in their 
modes of operation than IIR detecting devices. 

The second advantage is the better haze and fog 
penetration by the IIR. An IIR communication or 
recognition system of average power has no greater 
range than the same type of system adapted for the 
NIR, but because of the curious behavior of the 
water-vapor bands responsible for the IIR atten¬ 
uation (Section 4.8), the range is less affected by 
hazy weather. Also a given increment of power 
increase produces more effect on range than in the 
NIR. Incidentally, the better frequency response 
of the PbS cell may make possible new approaches 
to the communication problem by the IIR which are 
not possible with the more sluggish TF cell oper¬ 
ating in the NIR. 

The second great military use of the IIR is in 
heat detection. For military targets only a few de¬ 
grees above the background temperature, the peak 
of the differential heat emission occurs near 10 p. 
Such targets may be detected with thermal detectors 
(Chapter 8) sensitive in the far infrared, between 
the water-vapor and carbon-dioxide bands at 8 and 
13 p. But there is still appreciable emission by such 
targets in the IIR. With a detector such as the 


cooled PbS cell, which seems to be over 100 times 
more responsive in its wavelength range than con¬ 
ventional bolometers, a target only a few degrees 
above background may still be detected, in spite 
of the limitation of the cell response to wavelengths 
less than 3.6 p. Indeed, the first PbS-cell heat-de¬ 
tection system was able to pick up a certain ship 
target at about half the limit range of a well- 
developed thermal detector 11 (Chapter 9). For a 
new type of cell in the first apparatus built, this 
seems extremely promising. The German applica¬ 
tion of the cell in this manner is further proof of its 
potentialities. 

A great advantage of the PbS cell in this com¬ 
parison is its speed of response, the time constants 
being of the order of 10~ 4 second compared to 
2 X 10~ 3 second for the best thermal detectors devel¬ 
oped by NDRC during the war. The percentage of 
atmospheric transmission over a path length of 
several miles averaged over a wide band of wave¬ 
lengths in the IIR is no less, and may be greater, 
than in the FIR (Section 4.8). 

Description of Cells Developed 

Cell Types and Construction. Two main types of 
PbS cells were developed under Contract OEMsr- 
235. 10 The first is similar to the TF cell type shown 
in Figure 6, with the evaporated PbS layer placed 
on the bulb wall. The need for grid-shaped contacts 
is not so great as with TF cells since the resistivity 
of lead sulfide is less than that of thallous sulfide. 
Some PbS cells were made with grids, some simply 
between linear contacts. The grids or lines are ruled, 
like those in the TF cells, with Aquadag. 

The second type of PbS cell is like that shown in 
Figure 17. A re-entrant inner chamber permits cool¬ 
ing of the sensitive evaporated layer which is on 
the outside wall of this chamber. The leads are 
brought out from the bottom to minimize the prob¬ 
lem of current leakage due to condensed water vapor 
between the leads. A variant of this design was used 
for some cells which were made by chemical depo¬ 
sition of the PbS instead of evaporation. 

Various sizes of sensitive area have been made 
with good success from about 0.5 square millimeter 
to about 2 square inches. 

The cell envelopes are Nonex glass, though quartz 
would withstand high evaporating temperatures 
better and would give better transmission near the 
long-wavelength limit of the cell response beyond 






86 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


3 p, if it could be adapted to easy manufacture. 
Soft glass can be used for chemically deposited cells. 

The construction and cleaning of the glass com¬ 
ponents follow the lines already described for the 
TF cell in Section 3.3.1. 

The chemically deposited lead sulfide layers were 
made by Briickmann’s method from a reaction be¬ 
tween lead acetate, thiourea, and sodium hydroxide. 
This material had the same long-wavelength thresh¬ 
old and the same good filter transmission and fre¬ 
quency response as the evaporated PbS cells, but it 
never gave as high or uniform sensitivity as the lat¬ 
ter, although attempts were made to activate it by 
several different methods. 

The lead sulfide used for the evaporated layers is 
made by precipitation with H 2 S from a solution of 
lead nitrate in distilled water. The black precipitate 
is dried in a vacuum desiccator and then removed 
and fused in vacuum, like the T1 2 S for TF cells. 
The crystalline PbS is then crushed to a fine powder 
and stored in evacuated ampoules for future use. 

The evaporation procedure is similar to that for 
TF cells. About 10 milligrams of the PbS powder 
are required for the usual cell sizes. The conden¬ 
sation is directed to the proper part of the cell wall 
by suitably located external heaters and air blasts 
in addition to the gas flame used for the evaporation. 
The evaporation is carried out in the presence of 
about 200 microns pressure of air flowing contin¬ 
uously through the cell to sweep out S0 2 formed by 
reduction of some of the PbS. After the layer has 
been formed it is baked for 10 minutes at about 400 
C in an oven, then cooled to room temperature and 
the air flow stopped. Pumping continues for 10 to 30 
minutes during which the resistance increases to 
an apparently stable value. The cell is then sealed 
off. 

Summary of Characteristics. Some of the PbS 
cell characteristics are summarized in Table 1. The 
values are only rough indications of the cell possi¬ 
bilities, as they are based on only a very few cells 
constructed in ari initial stage of the development. 

An attempt is made to keep the cell resistance in 
the range from 0.5 to 20 megohms for the same rea¬ 
sons as given for the TF cell. Since the resistance 
increases by a factor of 10 to 100 on going to dry-ice 
temperature, cells which are to be cooled are made 
to have very low resistance, 0.2 megohm or less, at 
room temperatures. The temperature coefficient of 
resistance of the PbS cell is about 2 per cent per 


degree centigrade. This is about one-third that of 
the TF cell, a-s expected from the three times longer 
wavelength threshold (see “Variation of Resistance 
with Temperature,” Section 3.3.1). No consistent 
and detailed measurements of the change with tem¬ 
perature over a large range are yet available for 
computing the thermal activation energy of the 
PbS cell. 



Figure 18. Spectral response curves of some PbS 
cells. 


The spectral response curves of three PbS cells 
are shown in Figure 18. They were measured under 
Contract OEMsr-235 on a Bellingham and Stanley 
single-prism rock-salt monochromator. 10 The long- 
wavelength tail response shapes are uncertain be¬ 
cause of the large spectral slit widths required in 
the instrument at these wavelengths. For accurate 
threshold measurements, a doubly dispersing mono¬ 
chromator would be needed. The threshold is at ap¬ 
proximately 3.6 p just as in the German PbS cells. 
The peak near 2.5 p in each of the curves is believed 
to be characteristic of lead sulfide. The peak indi¬ 
cated near 1 u seems to have an intensity propor¬ 
tional to the amount of oxygen taken up by the cell. 
The response at wavelengths longer than 1 p de¬ 
pends greatly on the thickness of the layer used, 
for the material becomes almost transparent as the 
threshold is approached. Unfortunately the thicker 
layers are hard to sensitize, and the best surfaces 
seem to be slightly transparent even in the visible 
region. The spectral response has been measured to 





















PHOTOCONDUCTIVE CELLS 


87 


wavelengths shorter than 0.5 ^ with a Van Cittert 
glass double monochromator, and it is believed to 
continue to a short-wave limit imposed by the ab¬ 
sorption of the cell wall material. 

There are indications that the spectral response 
curve is not quite the same for cooled and uncooled 
cells. More careful measurements on this point are 
needed. 

The usefulness of the PbS cell in HR communica¬ 
tion and signaling devices depends not only on its 
spectral response curves but also on the transmis¬ 
sion of the atmosphere and optical materials and on 
the sources used. These factors are discussed in 
Section 4.8. 

At room temperature the S/N ratio of PbS cells 
made in 1945, as measured on the test set, is some 20 
to 30 db below that of a TF cell with a sensitive 
area of the same size if the tungsten light source is 
unfiltered in both cases. The Corning 2540 filter re¬ 
duces the TF response some 15 db, the PbS response 
about 5 db, leaving the PbS cell with a ratio 10 to 
20 db below that of the TF cell for the filtered 
source. [If the source were at 1500 K instead of near 
3000 K, these relations would be completely al¬ 
tered (see Section 4.8).] 

For the test set source filtered in this way, the 
hololuminous threshold signal of two different sizes 
of PbS cell is shown in Table 1. With such a source 
and filter, the total energy that lies in the wave¬ 
length region of PbS response is about 7 times as 
great as that in the region of TF response. Thus, 
the S/N ratio, per unit incident energy near the 
wavelength of peak response of each cell, is perhaps 
25 to 35 db lower for the PbS cell than for the TF 
cell. This estimate is confirmed by the comparative 
response of the two types of cells to a monochro¬ 
matic source, such as the cesium-vapor lamp. 

When the PbS cell is cooled and the load resistor 
in the circuit properly matched at every tempera¬ 
ture, in the usual circuits the noise stays about con¬ 
stant, but the signal strongly increases. At dry-ice 
temperature, the improvement in S/N ratio may 
amount to 20 or 30 db; at liquid-air temperature, 
several decibels more. 

Within statistical uncertainty on the few cells 
made, the S/N ratio of the PbS cells is inversely 
proportional to the square root of the area just as 
expected theoretically for such detectors and as 
already confirmed experimentally for TF cells. 

The absolute values of both signal and noise of 


the PbS cells are very low compared to TF cells 
of the same size (Table 1) and therefore impose 
exacting requirements on amplifier design if the 
ultimate sensitivity is to be reached. The best de¬ 
signs seem to be similar to those of TF-cell ampli¬ 
fiers, wdth careful selection of components to mini¬ 
mize noise. 

The signal and a large part of the noise are pro¬ 
portional to applied voltage, as with the TF cell. 
But even at low frequencies voltages over 45 are 
generally required before the voltage-dependent 
part of the noise becomes larger than the Johnson 
noise. Careful studies have been made under Con¬ 
tract OEMsr-235 on the variation of noise with 
voltage. 10 

The equations developed for the variation of sig¬ 
nal and noise wdth frequency for TF cells also apply 
here in first approximation (Figures 19, 20, and 21) 



100 IOOO 


FREQUENCY IN C 

Figure 19. PbS cell signal and noise versus frequency 
at room temperature. 

but with time constants substantially shorter. The 
t’s for PbS cells are about 10“ 4 second at room tem¬ 
perature and 5 X 10" 3 second at dry-ice temperature, 
compared to about 10“ 2 second for a TF cell at room 
temperature. As a result of these shorter time con¬ 
stants, the signal response of some PbS cells may 
be down as little as 5 db at 20,000 cycles compared 
with its value at 30 cycles. 

A more exact theory of signal and noise will be 
needed in order to explain the resonance-type peak 
at 1,500 cycles in the lower corrected S/N curve 
shown in Figure 21. Similar peaks occur at other 
frequencies in TF cells exposed to steady background 
light. It is just possible that they may be due to the 
amplifying circuits and not to the cells, although 



















































88 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 



100 1000 


FREQUENCY IN C 



90 


~2 

*3 

® O 


it 

OC I 

is 


70 2 ° 

(rt tf) 


60 


55 


100 


1000 


FREQUENCY IN C 


Figure 20. PbS cell signal and noise versus fre- Figure 21. PbS cell S/N ratios at room temperature 

quency at —80 C. and at —80 C. 


great precautions were taken to get true cell charac¬ 
teristics in these measurements. 

Present PbS cells may be operated in direct sun¬ 
light with only 1 to 4 db loss in S/N ratio. The re¬ 
sponse is linear with incident flux, from the noise 
level up to a flux of about 0.01 him, corresponding 
to 30 or 40 footcandles of tungsten lamp illumina¬ 
tion. The linearity thus extends over a range of 
almost 10 7 in incident flux level. The comments 
made for TF cells on the origin of the nonlinearity 
also apply here. 

It seems likely that the short time constants and 
better frequency response, the 10- to 100-fold in¬ 
crease in the range of linearity of the PbS cell, and 
its 10- to 100-fold smaller sensitivity to background 
light, compared to the TF cell, are all intimately 
associated with its 10- to 100-fold smaller respon- 
sivity per unit energy (at room temperature) than 
the latter cell. An insensitive cell in general has 
these otherwise favorable characteristics, as dis¬ 
cussed under “Fundamental and Theoretical 
Studies/’ Section 3.3.1. If this is the explanation 
here, then any success in increasing the responsivity 
of the PbS cell will be offset somewhat by poorer 
performance in these respects. 

It is possible also that increased responsivity may 
not be possible, the attainable S/N ratio of long- 
wavelength detectors perhaps being inherently lower 


than that of short. This is one of a number of basic 
limitations, independent of specific cell material or 
postulated mechanism, which are suggested by vari¬ 
ous aspects of the behavior, both absolute and com¬ 
parative, of TF and PbS cells and other selective 
photodetectors. Such theoretical relations, if they 
exist, seem not to have been developed or set down 
anywhere explicitly as yet. They would probably 
run parallel to the similar S/N ratio and tempera¬ 
ture relations which are already well established 
for nonselective thermal detectors. 

Heat Detection. The sensitivity of a TF cell and 
of a PbS cell, cooled and uncooled, to targets at 
temperatures of a few degrees above background, 
is shown in Figures 15 and 22. The ordinate in these 
graphs is the total incident energy, integrated over 
all wavelengths, from a black body, which must fall 
on the cell in order to produce threshold response. 
The response behavior as shown by these graphs 
differs from that of thermal detectors. The PbS 
cell sensitivity increases steeply with increasing 
temperature of the black body above its surround¬ 
ings because higher temperature radiators are richer 
in the shorter wavelengths to which the PbS cell 
is more sensitive. For ordinary nonselective thermal 
detectors, the sensitivity per unit integrated energy 
is almost independent of the black-body tempera¬ 
ture. 


RSgEMCT ED 






































































































PHOTOCONDUCTIVE CELLS 


89 



Figure 22. Sensitivity of PbS cell at —80 C as a 
thermal detector. 


If the temperature of the body is within a few 
degrees of that of the surroundings, it can be 
proved 10 that the response per unit energy for the 
PbS cell also becomes independent of the body’s 
temperature and is determined only by the spectral 
response curve in the last few tenths of a micron 
near the long-wavelength threshold, as shown in 
Figure 23. The curves of Figures 15 and 22 must 
thus become horizontal at the point where they 
intersect the vertical axis. This point was at about 
5 X 10~ 6 watt for one cell with a ^x^-inch sensi¬ 
tive area with 5-cycle bandwidth (Figure 15). It 
is at 3 X 10~ 7 watt for another cell with a lxl-inch 
sensitive area at room temperature. For the latter 
cell at —80 C (Figure 22), it becomes about 
3 X 10" s watt. This is equal to the response of ther¬ 
mal detectors, within a small factor, as proved by 
the tests on the equipment described in Chapter 8. 11 
For sources, such as unshielded motor exhausts, 
which are more than about 60 degrees above the 
background the cooled small PbS cell will evidently 
become much more sensitive than the thermal de¬ 
tectors. 

For motor exhausts, indeed, the PbS cell might be 
a uniquely sensitive detector because much of the 
radiation from them is said to be concentrated in 


gaseous molecular emission bands, some of which 
fall in the region of greatest sensitivity of this cell. 
The problem and possible success of this kind of de¬ 
tection, coupled with the important related problem 
of transmission of the radiation through the at¬ 
mospheric water-vapor and C0 2 absorption bands, 
needs a great deal of careful study and has scarcely 
been touched to date. 

Cells for heat detection should be made in quartz 
because of the poor transmission of Nonex and most 
other glasses in the important PbS threshold region, 
where the overall detection curve reaches a maxi¬ 
mum, as shown in Figure 23. 



o —-—-- / —--—— 

1.2 1.4 1.6 IB 2 D 22 2.4 2.6 2B 3.0 3.2 3.4 3.6 3.8 


WAVELENGTH IN MICRONS 

Figure 23. Effective wavelength region of PbS cell 

sensitivity to black-body radiation. 

Study of Variables 

Some samples of galena were found pure enough 
to give satisfactory results in the evaporation 
method. However, since the percentage of impurity 
is widely variable from one sample to another, ga¬ 
lena is not recommended for production purposes. 

The lead sulfide layers prepared by chemical de¬ 
position exhibited slight photoconductivity, which 
improved greatly on cooling the layer. It also im¬ 
proved if the layer was subjected to heat treatment 
in a vacuum for up to 5 hours at temperatures near 
150 C. No improvement was obtained by heat-treat¬ 
ing in oxygen. 

The heat treatment in vacuum increased the re¬ 
sistance and the S/N ratio each about 100 times. 
If the heat treatment was carried to temperatures 
above 170 C, the layer lost its photosensitivity per¬ 
manently, and the color changed from the blue-gray 
of PbS to a dull brown. Further heat-treating in air 
or oxygen or in vacuum at temperatures up to 500 
C failed to restore the photoconductivity. ~ 





































































90 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


The chemically deposited layers could also be 
made very photosensitive by alternate heating and 
cooling, using a gas-air flame in direct contact with 
the layer; the brown color did not appear in this 
case. 

The loss of sensitivity on slow heating to above 
170 C was thought by those working under Contract 
OEMsr-235 to be due to the appearance of decom¬ 
position products of oxides or organic compounds 
which might have been present in small quantities 
in the layer. Perhaps the sudden heating is success¬ 
ful because it drives out such impurities before 
decomposition can occur. 

Oxidation is carried on during the evaporation. 
The air is flowed through the cell continuously 
rather than allowed to stagnate because there are 
indications that decomposition and reduction of 
PbS accompany the oxidation at least above 400 C, 
with formation of S and S0 2 vapor. It is necessary 
bo sweep out these vapor products, as it appears that 
S0 2 poisons the sensitization, possibly by combin¬ 
ing with the sulfide. 

The rate of the increase of resistance which oc¬ 
curs after the cells are cool, during the final pumping 
prior to sealing off, is thought to indicate the pres¬ 
ence of vapors not removed by the pump, which are 
entering into a surface reaction with the layer. 
These vapors might be water or sulfur, but the exact 
reactions are unknown. 

Water vapor proved very important in the oxi¬ 
dation of the TF cells to insure stability and low 
resistance. It is much less important here because 
both properties are easily obtained using dry oxygen 
in the sensitization. However, the activation of the 
PbS may be carried out at lower temperatures when 
water vapor is present. This is considered desirable 
because the PbS is rather strongly reduced at higher 
temperatures, and hence water vapor is customarily 
used in the cell manufacture. 

Almost no fundamental and theoretical studies 
have been carried out on the PbS cell and the mech¬ 
anism of its photosensitization. Nevertheless, the 
similarities between its behavior and that of the TF 
cell are so great as to provoke reflection at every 
point and to suggest that both must be encompassed 
by the same theory and the same mechanism. Per¬ 
haps other sulfides, such as the Ag 2 S and the Mo 2 S 
already studied, may be made into successful cells 
with similar characteristics if further work is car¬ 
ried out on them. Other compounds, particularly the 
selenides and tellurides, also need study. Selective 


detectors sensitive to still longer wavelengths may 
be found by such investigation. 

Present Status 

The work begun under Contract OEMsr-235 is 
being continued at Northwestern University under 
Navy Contract NObs-25392, with particular em¬ 
phasis on the investigation, fundamental study, and 
development of PbS cells and other HR detectors. 

Recommendations 

The study and development of PbS cells can and 
should be brought up to and beyond the level at¬ 
tained with TF cells. Possibly the S/N ratios can 
be increased further. Those working under Contract 
OEMsr-235 have recommended more study of the 
effect of cooling chemically deposited PbS layers. 
More accurate spectral response data are needed, as 
well as measurements on signal and noise above 
20,000 cycles (for possible use in multichannel 
systems). 

The field of HR detectors and military systems 
seems wide open for further exploration and for 
theoretical analysis. Since the importance of this 
field is appreciated by the Armed Services and work 
is going forward in this direction, no further recom¬ 
mendations need be made. 

3,3,3 Silicon Cells 

Course of Development 

Work had been under way at the Bell Telephone 
Laboratories for some time on the preparation of 
semiconducting layers by the decomposition, at 
heated solid surfaces, or chlorides, hydrides, and 
other volatile compounds of certain elements. In 
1943, G. K. Teal and J. R. Fisher discovered that 
some silicon samples, prepared in this way by de¬ 
composing SiCl 4 , were photosensitive. 

Several small rod-shaped samples were tested by 
the University of Michigan Contract NDCrc-185, 
and proved so promising as infrared detectors that 
Contract OEMsr-1231 was set up in October 1943 
at Bell Laboratories with the principal object of 
extending these techniques to the production of 
silicon cells of larger size. 15 * 16 

Under this contract over 100 cells, each of an area 
of about 2 square inches, were made on porcelain 
or quartz backing plates and tested at Bell Labora¬ 
tories and at the University of Michigan to ascer- 


<j@gesEE££D 




PHOTOCONDUCTIVE CELLS 


91 


tain the effect of variables in the manufacturing 
process and to determine the order of magnitude of 
attainable responsivities. The cells compared favor¬ 
ably with TF cells in relative infrared response, in 
frequency response, and in their insensitivity to 
background light, but the attainable S/N ratios of 
the best cells, as measured on the photocell test set, 
were 30 to 50 db lower than those of average TF 
cells. 

It was concluded that the preparation of silicon 
cells which would be as successful for infrared de¬ 
tection as TF cells might not be impossible, but that 
it would require a long-time research program to 
accomplish. Because of the meager promise of suc¬ 
cess in a short time, Contract OEMsr-1231 was 
terminated in March 1944. 

Construction of Cells 

The cells were constructed by placing the porce¬ 
lain or quartz plates in a vacuum furnace through 
which SiCl 4 vapor was carried by a current of 
hydrogen gas. In the early experiments with small 
cells, only the cylindrical cell itself was heated by 
direct means, but to produce uniform large cells it 
was deemed necessary to heat the cell base disks 
indirectly in a furnace. It is believed that this may 
have introduced into the process an element of un¬ 
certainty about the exact effective temperature, 
pressure, and concentration of the reacting vapor. 

The SiCl 4 vapor was obtained by passing H 2 gas 
through a reflux condenser over a boiling flask of 
the liquid. This vapor mixture was diluted as de¬ 
sired by additional H 2 gas. In the studies, the SiCl 4 
concentration reaching the furnace was varied over 
wide limits, though the absolute concentration was 
indeterminate because of the effect of the heated 
furnace walls. The temperature of the furnace and 
cell during deposition was varied from 1017 to 1225 
C, and the time of deposition from 3 to 120 minutes, 
the best results being obtained at about 1030 C and 
20 minutes. 

The most sensitive films were not the most uni¬ 
form in appearance but were discolored with brown 
patches down the center of the plate. 

General Properties 

Only films with fairly high resistances—from 1 to 
20 megohms—were highly sensitive. 

The spectral response curve of these cells has a 
peak near 0.8500 \i and a threshold between 1.2 and 
1.4 p. 15 The ehT° values of various NIR filters 


with respect to these cells are usually equal to or a 
few per cent greater than the ehT° values with re¬ 
spect to TF cells. 

The noise levels for the Si cells as measured on 
the photocell test set were comparable with those 
of type A TF cells, but the signal response was 30 
to 50 db lower. Consequently, the signal equivalent 
of noise was very large—about 0.1 microhololumen 
at 90 cycles, with bandwidth 1.8 cycles, compared 
to about 10" 3 microhololumen for a TF cell. The 
response over a single Si cell surface was very non- 
uniform. 

In spite of this unfavorable comparison, the re- 
sponsivity of the small Si cells is reported to be 
equal to that of a selenium bridge photocell, and 
their stability and speed of response far better. 16 

Signal and, presumably, noise vary with fre¬ 
quency in the same way as with TF cells (see Sec¬ 
tion 3.3.1), but with much shorter time constants— 
about 3 X 10" 4 second, compared to about 10“ 2 sec¬ 
ond for TF cells. The signal response is thus down 
only 6 db at about 700 cycles. No careful measure¬ 
ments of the variation of noise with frequency have 
been reported. 

The response of the Si cell to modulated radia¬ 
tion is decreased less by steady background mask¬ 
ing flux than is the response of a TF cell. The Si 
cell response decreases by a factor of 4 under back¬ 
ground illumination of 500 footcandles, while TF- 
cell response may decrease by a factor of 100 or 
more under the same conditions, depending on cir¬ 
cuit adjustments. 

Very likely the short time constants and the in¬ 
sensitivity to background light are associated with 
an exceptional linearity of response, but this has 
not yet been reported; and all these properties are 
possibly bound up with the low responsivity just 
as was conjectured to be the case with PbS cells. 

The variation of resistance with temperature has 
been explained 16 in terms of three different appar¬ 
ent activation energies, one near 0 ev at low tem¬ 
peratures, another between 0.3 and 0.8 ev at 
temperatures from 200 to 500 C, and another of 
1.12 ev at high temperatures. It has been assumed 
that “the value 1.12 ev represents the separation of 
the conducting and nonconducting bands in sili¬ 
con.” 16 The Si films completely absorb radiation 
out to about 1.05 \i and become somewhat trans¬ 
parent at longer wavelengths. It is concluded that 
the same electron bands are concerned in the photo¬ 
electric, optical, and thermal processes; a similar 




92 


NON-IMAGE-FORMING NEAR INFRARED DETECTING DEVICES 


conclusion has already been reported in Section 
3.3.1 for TF cells. 

It was found under Contract OEMsr-1231 that 
the quality of Si cells produced was quite erratic, 
even when the conditions of deposition were con¬ 
trolled as carefully as possible, and that some uni¬ 
dentified variable must be present. It seems at least 
possible, judging from the similar early results with 
TF and PbS cells, that the variable may be oxida¬ 
tion. It is to be regretted that no direct attempts 
were made to photosensitize the Si films with oxy¬ 
gen, or oxygen and water vapor, or by some other 
method, following deposition. Perhaps this omission 
may be attributed in part to wartime security bar¬ 
riers to the free exchange of ideas and information. 
Since the Si films behave so nearly like the TF and 
PbS films in other respects, they may behave alike 
in this. 

It would seem to be worth while to continue the 
work with Si cells for the sake of the light they can 
throw on the nature of the photoconductive process. 

Present Status 

Contract OEMsr-1231 w^as terminated in March 
1944. Some work on Si cells seems to be continuing 
at Bell Laboratories. 16 

Recommendations 

These cells need more study for the comparison 
of their photoconductive mechanisms with those of 
TF and PbS cells. 

Selenium Electrolytic Cells 

Early in the war a quite different type of photo¬ 
conductive cell, a selenium electrolytic photocell, 
was developed by the Massachusetts Institute of 
Technology under Contract OEMsr-561 18 > 19 > 20 but 
without any very valuable military results. This 
contract was set up under Section D-3 in July 1942, 
and only the last few months of the contract, up to 
June 1943, were under Section 16.4. The cell is not 
especially suited for infrared work as it has a spec¬ 
tral response curve similar to that of the selenium 
barrier-layer cells in common use in footcandle 
meters and exposure meters with the peak response 
at about 5500 A and the long-wavelength threshold 
near 9000 A. Therefore only a brief description 
will be given here. 


The cell usually consists of platinum electrodes 
completely electroplated with metallic selenium and 
immersed in selenious acid. No gases are evolved 
and the acid renews itself during cell operation so 
that the cell can be hermetically sealed. With an 
external d-c potential of 2 to 4 volts across the cell, 
the cell resistance is found to vary with light inci¬ 
dent on the cathode. Load resistors of only a few 
thousand ohms are used or else equivalent feedback 
circuits. 

The cell has a short-circuit sensitivity of about 
1,000 pa per lumen with 2 volts applied, and a dark 
current after initial seasoning of about 1 pa, with 
noise about 10" 3 pa measured on a sensitive gal¬ 
vanometer. The signal equivalent of noise is thus, 
at best, about 1 phlm for light either unmodulated 
or modulated at 90 cycles. This is at least 1,000 
times less sensitive than a TF cell, for example. 

After termination of the contract, some of the 
personnel and facilities w r ere taken over by MIT 
Contract OEMsr-1036 which was set up at that 
time to study the fundamental properties of TF cells 
(Section 3.3.1). No further military development of 
this selenium cell was considered warranted. 

Summary of Recommendations 

Continuation by the Armed Services of the devel¬ 
opment and production of photoconductive cells, 
especially TF, PbS, and related types for the HR, 
is a “must” item for any infrared communication, 
signaling, and detection program. These cells are of 
the greatest military importance for ship, plane, or 
personnel detectors; for tracking, guiding, and heat¬ 
homing systems; for proximity fuzes, and for voice 
and code secret IR communication and signaling. 

Photovoltaic cells for the NIR and IIR need fur¬ 
ther study. 

Commercial production and development of pho- 
toemissive devices will probably take care of most 
needs of the Services in that line, so that the in¬ 
tensively sponsored development will be needed 
only to meet the specialized requirements of par¬ 
ticular applications. However, the design and 
fabrication of high-quality photomultiplier tubes 
for large-scale use is a rather specialized process for 
which the maintenance of small, constant pilot pro¬ 
duction may be advantageous if future military 
applications of such devices are likely to be exten¬ 
sive. 




Chapter 4 

NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 

By John R. Platt a 


INTRODUCTION 

Object and Scope of Chapter 

ne of the principal military uses for the near 
infrared [NIR], 0.8 to 1.4 p, sources, filters, and 
photodetectors described in Chapters 1, 2, and 3 is 
their assembly into complete systems for voice and 
code communication on an invisible “beam of light.” 
Such systems are commonly called photophones, op¬ 
tical telephones, or the like. Voice systems based on 
several different principles were developed under 
NDRC auspices and will be described in this chap¬ 
ter. A few of these systems include provision for 
code operation. Systems which transmit code only 
will be taken up in the next chapter. 

The voice systems cover a considerable range of 
sizes and military applications. A hand-held system 
(type W) weighs 25 pounds, has a beam angle of 5 
degrees and an average clear weather [ACW] night 
range of 3 miles (see Sections 4.3.2 and 4.3.1). A 
system for aircraft weighs 60 pounds, has a beam 
angle of 15 degrees, and an ACW range of 4.5 miles 
(Section 4.4.3). A shipboard installation (type E) 
weighs 200 pounds, has a beam angle of 15 degrees, 
and an ACW voice range of 6.5 nautical miles but 
a code range of 9 nautical miles. In these systems 
the intensity of the light beam is modulated at audio 
frequencies. As it was feared that after a time such 
systems might be in danger of being received by 
enemy infrared receivers occasionally, work was also 
undertaken on devices which could offer still more 
security. One such device made use of high-fre¬ 
quency carrier waves and one used modulated po¬ 
larization of the light beam. Another which has 
been considered would produce modulation by 
varying the wavelengths used for the communica¬ 
tion. 

Comparative mention will also be made of for¬ 
eign and of American NIR voice systems which 
were not developed under NDRC auspices. 

Estimates will be given of the expected perform- 

a Northwestern University, Evanston, Ill. Now at Uni¬ 
versity of Chicago, Chicago, Ill. 


ance of similar systems in the intermediate infrared 
[IIR], 1.4 to 6 p, which would at present have some¬ 
what more military security. 

4,1,2 Communication by Means of Near 
Infrared Radiation 

Advantages of the Near Infrared Region 

In principle, radiation of any wavelength region 
throughout the electromagnetic spectrum may be 
used for conveying energy in communication. Radio 
wavelengths are, of course, excellent for most uses, 
but for some military purposes they may have a 
dangerously great communication range and are 
subject to skip-distance phenomena and reception 
out of the line of sight even when ultrahigh fre¬ 
quencies are used in directed beams. For increased 
secrecy in combat communication it becomes desir¬ 
able to go to still shorter wavelengths for which the 
beam can be sharply defined and the range defi¬ 
nitely limited by the horizon or by atmospheric 
attenuation. 30 

At wavelengths shorter than the radar microwave 
region, radiation is not detected by tuned receivers 
but by thermoelectric elements in the region of long 
heat waves and by photoelectric detectors in the 
IIR region and at shorter wavelengths. Of these two 
types of detectors only the photoelectric devices 
have response times short enough to be used for 
receiving the audio-frequency variations in voice 
communication, and such communication is thus 
limited at present to wavelengths below 6 p. 

Optical communication systems require much 
higher powered transmitters than radio systems to 
obtain the same range for the same beam spread, 
even when atmospheric attenuation is neglected. 
This is partly because the effective “antenna” areas 
of practical photodetectors are much smaller than 
for radio; and partly because, even for equal re¬ 
ceived energy, photodetectors are less sensitive. This 
is because of the higher quantum energy required at 
optical frequencies to eject a photoelectron as com¬ 
pared with the quantum energy required at radio 




93 



94 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


frequencies to set an electron in motion in an an¬ 
tenna. A narrow-angle optical system may, however, 
compare in range with a nondirectional radio system 
of the same power. 

Sensitive photodetectors for the HR region, from 
6 to 1.4 [i, have been developed quite recently, and 
most work up to the present has been done in the 
NIR region, from 1.4 to 0.8 p. In the visible region, 
between 0.8 and 0.35 jx, optical systems are limited 
to daylight use where a flash of light will not jeop¬ 
ardize security. Even in the ultraviolet [UV], below 
0.35 p, night use is risky, because although the UV 
radiation itself cannot be seen at night it will pro¬ 
duce a strong and very visible fluorescence. In addi¬ 
tion, as the UV is approached, atmospheric attenua¬ 
tion becomes serious. The development by the 
NDRC of UV systems is reported in the Summary 
Technical Report of Division 16, Volume 4, Chap¬ 
ter 6. 

Considering these limitations on other spectral 
regions and the present American and foreign mili¬ 
tary interest in the NIR and IIR regions for various 
purposes, it seems likely that systems using these 
wavelengths will become increasingly important in 
future military short-range communication (under 
10 miles), for instance between tanks, advanced in¬ 
fantry, and ships and planes in convoy or formation. 
One drawback of such systems at present is their 
loss of range in daylight, but recent photodetector 
developments promise great improvement in this 
respect. 

Types of Military Applications 

Almost all earlier NIR systems used very narrow 
beams (under 1 degree) for high security. These re¬ 
quired fixed stations and tripod mounts and were 
unsuited to a mobile war. Wide-angle systems, with 
beams between 5 and 30 degrees, and even all- 
around systems with beams over 100 degrees wide 
give an intermediate security which still is much 
greater than that of radio. They can be hand-guided 
or in some cases need not be guided at all. 

The military characteristics required in the proj¬ 
ect control numbers of the developments to be 
reported show rather well the most urgent military 
problems for which such NIR systems were desired. 
All these problems involve communication between 
mobile positions or moving units at short ranges, 
under conditions requiring radio and radar silence. 

It may be noted in this connection that the use 


of the NIR system as a link between telephone lines 
across breaks or bad terrain, which has been a com¬ 
mon feature in narrow-beam systems for land sig¬ 
naling, was not requested in any of the NDRC proj¬ 
ects. 

A comparison of the weights, angles, and ranges 
of the systems developed under NDRC auspices 
with those of earlier systems is given in Figure 1. 

Ship Use. The type E system described in Section 
4.4.2 was developed for secret night communication 
by voice within convoys at ACW distances up to 
6.5 sea miles, code to 9 sea miles, with beam angles 
near 15 degrees, so that one ship could communicate 
with several others simultaneously. Its weight and 
power are compatible with ship operation. It can 
also be used for ship-to-shore harbor entrance com¬ 
munication. 

The Touvet system described in Section 4.5.3 
could be similarly used. It has similar voice range, 
with transmitter angle of 25 degrees and receiver 
angle of 2 degrees. It is an r-f system and offers a 
choice of six carrier-wave channels. 

The type G system (not an NDRC development) 
described in Section 4.3.1 could be used for the same 
problem, but it has a narrow beam of 1 to 4 degrees 
and, consequently, more security against detection. 
It is to be mounted on a stabilized platform and 
guided by an auxiliary tracking system and is to 
have an ACW range of about 4 miles. 

Aircraft Use. The plane-to-plane [P-P] system 
described in Section 4.4.4 was developed for secret 
communication between adjacent planes of B-29 
bombing formations during the hours of approach to 
the target. The system was not completed because 
of the termination of the war, but it was designed 
to give angles of 120 degrees horizontally by 60 
degrees vertically at the front and also at the rear 
of a bomber, with estimated ACW ranges of ^ niile 
in the daytime or 2 miles at night (with different cir¬ 
cuits). Weight and power conform to Army Air 
Force requests. 

The plane-to-ground [P-G] system described in 
Section 4.4.3 is for use in situations involving the 
maintenance of communications and transport of 
supplies by air at night to guerrillas, paratroops, or 
other isolated ground troops. The complete ground 
unit (type W, Section 4.3.2) weighs about 25 pounds 
and can be carried to earth on the person of a para¬ 
trooper. The complete plane installation of the P-G 
system would weigh 60 pounds and the transceiver 



INTRODUCTION 


95 


SYSTEM 


w 


RANGE-DIRECTIVITY PATTERN TOTAL 

WEIGHT 


2 SEA MILES 


5° 

HO 

3 SEA MILES 


(TWO INTERCHANGEABLE WIDTHS) SAME RANGE 
AS TYPE P“G 


20 LB 



PG 


P-P 




4.5 SEA MILES 
SAME RANGE AS TYPE W 


60 LB 


NIGHT: 2 SEA MILES 105 LB 

DAY: 0.5 SEA MILES (3 LAMPS) 



PH OTOE LAST 1C 
SHUTTER(RF) 
(SECRET) 



4 SEA MILES 


COMPARISON SYSTEMS 


A U RAL 
S IGN AL 



2 SEA MILES 


R-2 

OPTIPHONE 


l * 

LIGHT TELEPHON E i 15 
(JAPANESE) 


1.6 SEA MILES 



rT 


1.3 SEA MILES 


NIGHT.* 5 SEA MILES 
DAY ; 3 SEA MILES 


600 LB 


OVER 60 LB 


35 LB 
(TPOWER) 

49 LB 
(+ POWER) 

145 LBS* 


110 LB* 
(+ POWER) 


Li 250/130 
(GERMAN) 



Figure 1 . ACW ranges and angles of communication systems. 


All patterns show ACW ranges between two similar systems, of which only one is shown. They are night-voice ranges except as indicated. 
Receiver and transmitter patterns are roughly identical except as noted. The P-P pattern is for planes flying parallel, but all other plots show 
the maximum range when the second system is pointing directly at the system shown. The starred weights refer to units produced to specifications 
for field use, which may be several times heavier than laboratory models of the same type. 






















96 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


would be hand-operated through a hatch door. The 
beam angles are about 15 degrees for the plane unit 
and 10 degrees for the ground unit. These are suffi¬ 
cient for making contact and maintaining com¬ 
munication between the hand-held units, which are 
guided with the help of auxiliary infrared electron 
telescopes. The ground-to-plane ACW range should 
be about 2 miles, plane-to-ground about 4 miles. 

Ground TJse. The ground unit of the plane-to- 
ground system just described was adapted from an 
ultraviolet hand-held unit developed for daylight 
communication between landing barges, with angles 
of 5 degrees and ranges up to 2 miles. The NIR 
adaptation (type W, Section 4.3.2) gives ACW 
ranges of 3 miles with the same beam angle and is 
most suitable for night communication. Apparently, 
NIR and UV sources and cells can be made inter¬ 
changeable. Type W can be operated from the self- 
contained power supply for at least one hour. For 
longer operation, it may be connected to a 6-volt 
storage battery. 

Mixed Use. If NIR systems become more com¬ 
mon, communication between units of different 
types may become as usual as with radio. Most of 
the systems to be described will transmit to and 
receive from each other interchangeably. 

In addition to the ship-to-shore and plane-to- 
ground arrangements described, there are other com¬ 
binations. One might be ship to landing barge or 
beachhead, using type E, or preferably the stabilized 
narrow-beam type G on the ship and type W on the 
shore. Expected ACW ranges might be: type W (5 
degrees) to type E, 3 sea miles; type E to type W, 
4.5 sea miles. 

Another combination would be ship to plane, from 
type E (which has high elevation angles) on the ship 
to the P-G system on the plane. The ACW ranges 
in either direction would be about 5 sea miles. 

Since type W is portable and can be operated 
from a 6-volt battery, it is not restricted to ground 
use but may be carried on ships, planes, and tanks 
for short-range communication. 

Various modifications of these systems have been 
proposed to give increased security in each of these 
applications, if and when enemy interception of 
messages should become a troublesome problem. It 
must be remembered that in most cases the added 
security is obtained at the expense of lowered effi¬ 
ciency and increased complexity. 

Identification and Recognition. Another impor¬ 


tant military application of infrared radiation is the 
equipping of military units—ships, planes, tanks, 
infantry groups, trench positions—with secret, all- 
around view, continuously operated beacons broad¬ 
casting a unique signal, and with directed receivers 
so that the units are immediately identified as 
friendly by any other similarly equipped unit within 
range. Some systems designed exclusively for this 
purpose will be described in Chapter 5, but it should 
be pointed out here that the voice systems of the 
present chapter usually contain all the apparatus 
necessary for this purpose and can be adapted elec¬ 
trically with a minimum of revisions to do both jobs. 
Optically, the transmitters are not so well suited to 
do both jobs, as only the P-P system (Section 4.4.4) 
has the wide angle usually required for identifica¬ 
tion from all directions. However, the narrow-angle 
voice receivers could be used for search purposes, 
like the type D recognition system receivers de¬ 
scribed in Section 5.2, by sweeping them automati¬ 
cally and continuously around the horizon when 
they are not being used for communication. Further 
remarks on the duplication and overlapping of func¬ 
tions of the various systems will be found in Sec¬ 
tion 5.2. 

The identification function was considered in the 
original request for development of the P-P system. 
The transmitters were to broadcast a code tone con¬ 
tinuously when not communicating, and this tone 
was to be picked up on other planes by narrow- 
angle receivers bore-sighted with the guns so as to 
provide a warning signal when they were turned 
on a friendly plane. This function was later elim¬ 
inated from the P-P system as the AAF decided to 
use another system for identification, but it could 
probably be performed by the P-P system with 
very little increase in weight or complexity. 

Types of Systems 

A beam of light or radiation has three properties: 
intensity, wavelength distribution, and polarization. 
The variation of any one of these by suitable modu¬ 
lation may be employed as the basis of a voice com¬ 
munication system. In a rough way, variation of 
intensity corresponds to amplitude-modulation 
[AM] in radio and variation of wavelength distri¬ 
bution to frequency-modulation [FM], although the 
analogies are not exact, since the receiver is not 
tuned to the “optical” frequency. 

The desired property of the radiation is modu- 




INTRODUCTION 


97 


lated at lower audio or radio frequencies (a-f or r-f). 
Simple a-f modulation involves AM in the modu¬ 
lating and receiving circuits, and systems of this 
kind will be called amplitude-modulation systems, 
even though the property of the radiation beam 
which is modulated may not be the optical ampli¬ 
tude. If photodetectors with very short response 
times are used, another kind of modulation may be 
employed, consisting of an amplitude-modulated 
r-f carrier wave on which is superposed either an 
AM or an FM audio signal (see Table 1). 

is now used, although some earlier studies involved 
sound pressure modulation of a manometric acety¬ 
lene flame. For modulating the outgoing beam, 
electro-optical devices like the Kerr cell have been 
considered, but most military systems use mechani¬ 
cal modulation, with vibrating mirrors and beam- 
chopping devices in a-f systems or supersonic vibra¬ 
tions of some optical element in r-f systems. 

Some NIR optical communication systems are 
arranged in Table 1 according to the optical prop¬ 
erty being modulated and the kind of modulation. 

Table 1. Types of code and voice communication systems: (c) indicates code only; (v) voice only; a NCRC consul¬ 
tation ; s NDRC study; systems in italics, NDRC developments. 

1 

Kind of modulation 

2 

Intensity* 

3 4 

Optical property modulated 

Polarization Wavelength 

a-f (AM) 

Mechanically modulated 
(tungsten) 

Type D (c) 

Type G a 

Type R-2 

Type W (v) 

Light-beam telephone 
(Japanese) 

Li 50. Li 80, Li 250-130* 
(German) 

Spectral modulation system 
(proposed) 

Electrically modulated 

(tungsten) 

(gas) 

Type D-2 (c) 

ASE and others 
(British) 

F.F.115 and others 
(Italian) 

Type E 

Plane-plane (v) 
Plane-ground (v) 

Aural signal unit (v) 

Type L a 

r-f (AM) 

Mechanically modulated 

Optiphone (a-f receiver 
only) 

Photoelastic shutter 
system 

Electrically modulated 

V-M system 

Touvet system 


r-f (FM) 


* There are two other intensity-modulated systems which do not fit a code or teletype Navy-developed system, type P, which makes use of 

easily into this table. One is an ordinary blinker code system which may very high-frequency spaced pulses of polarized light (see Section 5.1.2). 

be regarded as very low-frequency amplitude modulation. The other is 


The modulation may be accomplished either by 
electrically modulating the radiation source or by 
using a constant-intensity source and modulating 
the outgoing beam. In the first case, electrical mod¬ 
ulation of the current through a filament lamp (a-f) 
or through a gaseous discharge source (a-f or r-f) 


The modulation technique may determine the size, 
power, efficiency, and security of the source and in 
some cases the design of the receiver. 

The large number of voice systems shown in col¬ 
umn 2 indicates that the intensity-modulated a-f 
type of system is the simplest in design and con- 















98 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


struction. Most of the components of such systems 
are commercially available. The mechanically mod¬ 
ulated systems are the lightest in weight; the elec¬ 
trically modulated ones can attain the highest 
efficiency of light utilization. 

Security. Security against reception of intelligence 
by the enemy is called message security ; security 
against detection by the enemy of the existence of a 
modulated infrared system is called system security. 
Since the systems of column 2 are numerous and 
simple, they have very little security of either kind 
when operated in close contact with the enemy. 
They have the NIR advantages of invisibility and 
limited range, and some also have the advantage of 
narrow-beam width, but they can all be received 
by any NIR a-f receiver. Speech-scrambling has not 
yet been used, but it could be incorporated in exist¬ 
ing systems to improve message security. It would 
not affect system security. 

American equipment for a-f detection of enemy 
NIR systems has been built (see Section 5.5). Pre¬ 
sumably, foreign equipment for a similar purpose 
may shortly begin to limit the usefulness of the 
American systems shown in column 2. When that 
occurs, it may be possible to change some of the 
electrically modulated gas discharge sources to r-f 
FM, or to r-f AM with superimposed d-c, and thus 
make them secure against a-f receivers. Or it may be 
possible to go to systems like those in columns 3 and 
4. Reception of intelligence from the transmitters 
of the systems in column 3 requires plane, or plane 
and quarter-wave, polarizing sheets over the re¬ 
ceiver. Reception of intelligence from the trans¬ 
mitter of the recently proposed system listed in 
column 4 is more complicated, although the system 
promises to be simple to construct. 

These added security devices all give message se¬ 
curity. Greater system security may be obtained at 
present by using another wavelength region, the HR. 
Work in the HR requires a new type of detector 
cell, so far not extensively used, and it prevents 
reception of the message or detection of the source 
by the various and common NIR detectors. 

The systems in columns 3 and 4 are necessarily 
less efficient than those in column 2 because of the 
additional complication of the equipment and the 
additional restrictions on the light beam introduced 
by the security devices. It is not yet certain whether 
the ranges of HR systems will be as great as those 
of closely similar NIR systems. 


4,1,3 Common Aspects of All Systems 1 * 
Principles of Operation 

The operation of all these systems may be under¬ 
stood by referring to Figure 2, which is a schematic 
diagram of the type E system. Sound energy is con¬ 
verted to electrical energy by the microphone at the 
transmitter and is amplified by the audio-frequency 
amplifier to a level capable of modulating the trans¬ 
mitter current. This may be the current supplied to 
the source itself, if it is electrically modulated like 
the source in Figure 2, or it may be the current 
supplied to a mirror or other optical element lo¬ 
cated in the transmitter beam emanating from a 



Figure 2. Block diagram of communication system. 

steady source. The intensity, polarization, or wave¬ 
length of the radiation leaving the transmitter may 
thus, by one or the other of these methods, be made 
to vary in proportion to the modulating audio cur¬ 
rent and in accordance with the characteristics of 
the original sound. The reflector shown in Figure 2 
may be replaced in other systems by a more com¬ 
plex transmitter optical arrangement. The radiation 
is directed toward the distant receiver through an 
infrared filter designed to eliminate the visible por¬ 
tion of the spectrum. 

The small amount of infrared radiation reaching 
the receiver is concentrated by a mirror or lens upon 
the detector, wffiich is generally a photoconductive or 
photoemissive cell. If the modulation is in the wave¬ 
length or polarization of the beam, it is converted 
to an intensity variation by a suitable device. The 
photocell current varies in magnitude with the in¬ 
tensity of radiation falling upon the cell, and the 
fluctuating current is a fairly accurate copy of the 
current modulated by the original sound at the 
transmitter. The fluctuating current is usually 

b For list of symbols used in the equations of this sec¬ 
tion, see end of chapter. 


RfedLJiHrThh 




















INTRODUCTION 


99 


changed by passage through a load resistor to a 
fluctuating potential which is applied to the input of 
the receiver audio amplifier. The electrical energy 
available at the output of the amplifier is converted 
again to sound by the headphones or loud-speaker. 
That these numerous conversions of energy can be 
accomplished without excessive distortion is indi¬ 
cated by the fact that conversation can be carried 
on over a distance of several miles with high intelli¬ 
gibility using systems based on such principles. 

The Range Equation 

The range of such a system depends on many fac¬ 
tors such as the power and efficiency of the source, 
the responsivity of the detector, the beam angles, 
and the optical areas, but it depends most of all on 
the weather. 23 * 31 Knowledge of the relations between 
these variables is helpful in evaluating the compo¬ 
nents of a system or in predicting the overall per¬ 
formance of an untested system. The relations are 
formally independent of wavelength, and therefore 
the usual photometric (visible light) symbols may 
be used. 

The fundamental range equation is 

Rv 2 = l~, ( 1 ) 

where R v is the vacuum range of a system, or the 
maximum distance that the receiver could 
be separated from the transmitter and still 
obtain intelligible communication if no 
atmospheric attenuation were present (at 
night, unless otherwise noted), 

I is the maximum effective beam intensity 
(candlepower), 

A is the gathering area, or entrance pupil, 
of the receiver optical system, 

F is the effective threshold flux falling on the 
receiver which is required for just-intelli¬ 
gible communication. 

The equation can be derived from the primary 
definition of intensity I as the flux F per unit solid 
angle, since A/R 2 is the solid angle subtended at the 
transmitter by the receiver. This equation must be 
modified to take account of losses, and it can be re¬ 
written to express I and F in terms of other varia¬ 
bles. 

Atmospheric Attenuation 

Transmission. The most serious loss is the result 
of absorption and scattering by fog, smoke, and 


dust in the atmosphere. The absorption of two or 
three small bands of water vapor in the NIR may 
be neglected. In the NIR the transmission obeys 
Lambert’s law to a good approximation, so that if 
T is the fraction of light transmitted through unit 
distance of atmosphere (T < 1.0) then the fraction 
transmitted through R units is T R . 

The values of T for the visible and the NIR may 
be taken to be almost the same at any given time. 

Operational Range. The measured limiting oper¬ 
ational range R T of communication in weather with 
transmission T may be much less than the vacuum 
range R v . The illumination E in an attenuated beam 
falls off as 31 

E = ^ TB - ( 2 ) 

The threshold illumination on a receiver must be 
the same at R T for transmission T as at R v for 
transmission unity, or 

1 pR — I 

R t 2 R 2 ' 

and 

R t 2 T~ r = R 2 . (3) 

The value of R T as a function of R v is plotted for 
several values of T in Figure 3. In practice, R v must 
be computed from observed values of R T and T , or it 
can be determined from measurements in the lab¬ 
oratory. 



VACUUM RANGE R v IN YARDS XIO 3 

Figure 3. Atmospheric transmission and range. 

Average Clear Weather. As R v may be several 
times larger than actual ranges, it is convenient in 
comparing the performance of systems to specify 
another standard range, the range in average clear 





100 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


weather [ACW]. Usually ACW is taken by the 
Navy to refer to a transmission of 0.6 per sea mile, 
and this range would then be denoted by R 0 .e in 
equations (2) and (3). (The phrase “0.6 per sea 
mile” is the accepted substitute for the awkward 
exact expression, “0.6 1/mi,e ,” which is necessary if 
T R is to be dimensionless.) 

This value of T corresponds to a loss in signal of 
4.5 db per mile, in addition to the reduction by the 
inverse square law. A convenient rough rule is that 
the rate of loss in ACW, at distances over 5 miles, 
is about 6 db per mile. Thus a change by a factor of 
two in any of the variables determining range 
(beam intensity, detector responsivity, receiver 
area) changes the ACW range by about 1 mile. 

In ACW the daylight visible range, as defined 
later, is about 7 sea miles, and the code number 
is 7 (clear) in the International Visibility Code. 

While the ACW range is an easily interpreted 
index of the military performance of a system, it 
is not so satisfactory for laboratory or theoretical 
comparison of systems as the vacuum range. The 
ACW range involves arbitrary and not easily veri¬ 
fied assumptions about attenuation at the wave¬ 
lengths used, and computing it requires troublesome 
logarithmic conversions and reconversions. 31 The 
vacuum range should be more widely adopted as the 
measure of communication system performance. In 
this chapter, for systems for which the vacuum range 
has not been computed, an attempt will be made 
to estimate it so that the various systems may be 
more easily compared on an absolute basis. 

In order to compute R v or the ACW range from 
measured operational ranges, the value of T during 
the operation must be known. This may be found 
either from estimates of daylight visible range or 
from instrumental determinations. 

Daylight Visible Range. The daylight visible 
range, or limiting range at which large black objects 
can just be seen against a white sky, is proportional 
to 37 > 38 

1 

log (1/T)’ 

and varies more rapidly with T than does R T . The 
visible range in good weather is greater than the 
range of a communication system, becoming in¬ 
finite for transmission unity when the communica¬ 
tion range only becomes R v , but in murky weather it 
may be less than the communication range. The 


relation of T to the visible range and to the index 
numbers used in the International Visibility Code 
is shown in Figure 3. 

From visibility estimates values of T may be de¬ 
termined to an accuracy of about 0.1 per mile. Ob¬ 
viously, the daylight visible range can be used to 
determine T for a night operation only if the 
weather appears to remain almost unchanged in the 
interim. The visible range must not be confused 
with the night visual range [NVR] or range at 
which a filtered NIR transmitter can be detected 
by the dark-adapted eye. 

Instrumental Determination of T. Several differ¬ 
ent instrumental methods, none of them very satis¬ 
factory, have been used to determine T. The 
transmission may be found from absolute measure¬ 
ment of the flux received on a photocell from a fixed 
radiation source at a fixed distance of several miles. 
Or the signal level of communication between a 
shore station and a moving ship may be plotted 
against the distance, and the transmission deter¬ 
mined from equation (2). 16 > 17 A variation of the 
latter method is to use not the signal level but the 
brightness of the distant source (compared to a 
known local source) as observed in an optical tele¬ 
scope or electron telescope. Besides the obvious ex¬ 
perimental problems in such determinations, a 
fundamental difficulty is that the transmission over 
any path is continually changing and a consistent 
set of values is therefore rare; the spread of such 
determinations may be less than 0.1 per mile only 
if the transmission stays fairly constant. 

Some very rapid “twinkle effects” which have 
been reported as interfering with voice and code 
communication (see “Operational Tests” in Sections 
4.6.2 and 5.2) are probably of refractive origin 
rather than being due to true variations in T, but 
of course they further complicate the problem of 
measuring T. 

Transmitter Factors 

Steady sources and electrically modulated sources 
of radiation have already been described in Chap¬ 
ter 1. Methods of mechanical modulation will be 
described in the subsequent discussion of individual 
systems. 

Modulation. The maximum effective beam candle- 
power I, in equation (1), from a transmitter is pro¬ 
portional to the fraction z of the steady radiation 
which can be modulated by the impressed communi- 




INTRODUCTION 


101 


cation signal. Attainable values of z differ consider¬ 
ably for various modulation methods and for various 
types of sources. This difference is seen in Table 2, 


Table 2. Efficiencies of various modulation methods. 


Source and method 

Modulation efficiency 

Cesium-vapor lamp; electrically 

modulated (up to 5,000 c) 

2.00 

Concentrated-arc lamp; electri- 

cally modulated (1,000 c) 

0.30 

Tungsten lamp; electrically mod- 

ulated (1,000 c) 

0.30 

Vibrating mirror, opaque grid 

0.50 

Vibrating mirror, prism grid 

(Li 250) 

1.00 

Spectral modulation 

0.50 

Cesium lamp polarization system 

0.50 

(ideal; actual near 0.25) 

Supersonic diffraction (optiphone) 

r-f crest to trough 

1.00 

(ideal; actual near 0.70) 

a-f crest to trough 

0.50 

(ideal; actual near 0.35) 

Rare gas carrier-wave lamps 

r-f crest to trough 

2.00* 

a-f crest to trough 

1.00* 

Photoelastic shutter 

r-f, averaged over shutter 

0.35 

a-f, averaged over shutter 

0.25 

(ideal; actual near 0.06) 


* The intensity without the modulation device is taken to be the 
average intensity over an r-f cycle of maximum amplitude. 


which gives the modulation efficiency for various 
methods. Electrical modulation of the cesium-vapor 
and rare gas sources is seen to be the most efficient 
method. The modulation efficiency is here defined 
as the maximum crest-to-trough change of radiant 
intensity producible by an audio signal, relative to 
the steady unmodulated radiant intensity of the 
same beam (with the modulating device omitted in 
cases where it is external to the source and obstructs 
part of the beam). So defined, the term is applicable 
to all modulation methods; for electrically modu¬ 
lated sources, it is twice the modulation ratio defined 
in Chapter 1. 

The value of z mentioned above may be taken to 
be the relative rms variation of intensity with re¬ 
spect to the mean d-c intensity. For a maximum 
steady tone, z is M>\/2 times the modulation effi¬ 
ciency; for voice signal it is usually half or less of 
this value, depending on the transmitter circuits. 

Pass Band. Maximum communication range de¬ 
mands not only maximum modulation of the source 
light but also a suitable choice of the modulation 


frequencies. If the strong low frequencies of the 
human voice are allowed to pass through the trans¬ 
mitter amplifier, they may reach amplitudes of over- 
modulation while the high frequencies important for 
intelligibility are Still very weak. By cutting out 
the low frequencies in the amplifier, more energy 
may be put into the intelligibility frequencies with¬ 
out overmodulation. If the cutoff, for example, is at 
1,000 cycles, 86 per cent of the energy is removed, 
permitting an increase of 17 db in the modulation of 
high frequencies with a loss of only 7 per cent in 
intelligibility. 

This possible improvement has not been appre¬ 
ciated in many of the designs to be discussed, and 
some project control numbers have actually speci¬ 
fied voice pass bands from 100 to 1,000 cycles, which 
would give very much smaller communication 
ranges. The optimum band-pass for optical trans¬ 
mitters needs careful study similar to that which 
has been given sound-powered phones. 41 

Removal of the low frequencies of speech causes 
much of the naturalness and identifying character¬ 
istics of the individual voice to be lost. For military 
applications this is of little consequence. Other ways 
to increase the intelligibility output are discussed 
under “Amplification” in Section 4.4.2. 

Narrow-Beam Systems. The effective beam 
candlepower I depends also on the optical system. 
Consider first a collimating system with precision 
optical elements, that is, a system which focuses an 
image of the source at infinity. On looking back 
into such a system from the center of the beam at a 
great distance away, the exit pupil A t is seen to be 
completely filled and of brightness about equal to 
the surface brightness B of the source. Then the 
effective intensity is 

I = e/h 2 BA t , (4) 

where e{ takes account of the transmission and re¬ 
flection losses in the optical elements and h is the 
effective holotransmission [ehT] of any NIR filter 
in the beam. 43 If the system consists of a simple 
lens, or reflector, of focal length f, the solid angle 
Q t into which the beam goes is determined by the 
effective projected area of the source, af. 

Qt = fi (5) 

High Efficiency. Such a system may be made more 
efficient by increasing I or A t , with no change in 









102 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


focal length or Q*. Or it may be made more efficient 
by increasing Q* through a decrease in /, with no 
change in A or 7; in the latter case one way of 
utilizing the increased efficiency is by decreasing the 
source size and input power so as to restore the ini¬ 
tial beam solid angle. Both these cases reach a limit 
when the largest practicable fraction e t of the 
emergent flux qp from the source goes into the trans¬ 
mitter beam. The fraction e t may be called the 
transmitter efficiency. Then 

I = e,hzV (6) 

‘>&t 

For large values of e t (over about 0.25), reflectors 
must be used. The minimum area (exit pupil) A t 
required for them is given approximately by 

At — o t a 0 (7) 

where a 0 is the total luminous surface area of the 
source. 

Since in most communication systems the highest 
efficiency is desired, equation (6) is fundamental. 
It is applicable also to noncollimating systems and 
wide-angle systems generally, with or without beam¬ 
spreading devices. 

Usually, in the systems to be considered, the 
values of Q* were not so small nor the source areas 
a 0 so large as to make the efficient reflector size A t 
prohibitive. 

From the proportionality between a 0 and Q* in 
equation (7) we see the general rule for choosing 
the source for a given communication problem: wide 
angle, large source; narrow angle, small source. 

The total emergent flux qp from a source may be 
thought of as proportional to the product of the 
input power and the hololuminous efficiency in holo- 
lumens per watt. This efficiency is of the order of 15 
him per watt for all the sources considered here 
except the Western Union concentrated arc (Section 
4.4.2); for the latter, it is only 3 to 5 him per watt 
for arc sizes near 100 watts. 

Beam Solid Angle. Equation (6) implies that the 
beam has uniform intensity within angle Q* and 
zero outside. In practice, with most beam distribu¬ 
tions considered here, equation (6) holds approxi¬ 
mately if Q* is taken to be bounded by the directions 
in which the beam intensity falls to half the peak 
intensity [hpi]. Only hpi solid angles and beam 
widths will be used hereafter. 

The relation between I and Q* is of the greatest 


importance in the design of optical communication 
systems. To obtain maximum intensity and com¬ 
munication range with minimum source power, the 
beam must be as narrow as possible for the desired 
purpose. For a given source, power, and optical 
efficiency, wide-angle systems are short-range sys¬ 
tems. 

Laboratory Methods. Candlepower distributions 
may be measured by placing a source or a transmit¬ 
ter optical system, as the case may be, on a rotating 
table and determining the response of a fixed de¬ 
tector at some distance away as a function of angle. 
The distance away must be great enough that the 
angle subtended by the source at the detector is 
small compared to the hpi width. 

The ratio between the maximum candlepowers, I 
from a transmitter system and 7 0 from its bare 
source alone (both a-c or both d-c measurements), 
is the transmitter optics factor, O t 

I = IoO t . (6a) 

The vacuum range R v from a given transmitter to 
a given receiver may be determined in the labora¬ 
tory by finding the maximum communication range 
from the bare source to this receiver and multiply¬ 
ing this value by V O t . 

If the source is too intense to measure the latter 
range directly in the laboratory space, it may be 
measured indirectly by reducing the source intensity 
by a known amount as follows. 9 A lens or mirror is 
set up so as to form a reduced image of the source, 
and so that the receiver can “see” only this image. 
The intensity from this image is less than the inten¬ 
sity of the source by a factor p 2 /q 2 , where p and q 
are the object and image distances from the lens or 
mirror. The communication range from the image 
to the receiver is thus less by a factor p/q than the 
range from the bare source itself. 

Choice of Filter. Commonly, a maximum permis¬ 
sible NVR is specified in the requirements for an 
NIR system. The NVR is defined as the visual 
range limit of a transmitter to the dark-adapted 
standard eye in total darkness. It is determined by 
the kind of source, its holocandlepower, and the 
kind of filter, as discussed in Chapter 2. 

The specified NVR should be made as great as is 
militarily feasible because the larger the NVR is, 
the greater the operational range can be for a given 
transmitter system. This results from the fact that 
the ehT and the effective visual transmission [evT] 



INTRODUCTION 


103 


of a filter type vary together with changing optical 
density. Of course the filter type must be chosen 
from among those with satisfactory weathering 
properties and thermal and mechanical stability for 
military use. It should be a type which will make 
as great a differential as practicable between the 
ACW range and the NVR. This means it must have 
a high index of merit (see Chapter 2) if this can be 
obtained in conjunction with a reasonable ehT so 
that the input power needed to obtain the desired 
ACW range will not be excessive. 

With a specified NVR and a chosen filter type, 
the thickness or density must then be chosen to 
bring the NVR right up to the specified limit so as 
to make the communication range as great as pos¬ 
sible. 

The NVR for a given transmitter may be deter¬ 
mined by computations for the standard eye accord¬ 
ing to the methods outlined in Chapter 2, or it may 
be determined for actual observers in the labora¬ 
tory, reducing the range by a known amount as de¬ 
scribed above under “Laboratory Methods,” so 
that the test may be accommodated in a limited 
laboratory space. 

Receiver Factors 

The contribution of the receiver to the range is 
represented by the term A/F in equation (1). 

Effective Threshold Flux (Signal Equivalent of 
Noise) . The value of F depends on the spectral dis¬ 
tribution of the radiation and on the spectral re¬ 
sponse of the detector cell. All other factors being 
equal, the type of cell chosen must naturally be that 
giving a maximum signal-to-noise [S/N] ratio for 
the given spectral distribution and audio- or radio¬ 
frequency distribution of the modulated radiation 
from the filtered NIR source. What the best source 
or filter is also depends on the cell; actually no one 
of them should be chosen independently, but all 
combinations should be studied to find the best for 
a particular problem. In addition, the value of F 
depends upon what fraction e r of the flux falling on 
the receiver entrance pupil actually reaches the 
photodetector cell. 

The threshold flux is determined by the limitation 
of intelligibility because of the noise in the cell and 
circuit. In circuits with low noise, the noise is gen¬ 
erally a function of the effective cell area a and 
receiver band width A/, as explained in Chapter 3. 
The ratio of the rms threshold signal to the rms 


noise, threshold S/N, depends on whether the com¬ 
munication is code or voice, on the kind of source 
and cell, and the circuits. 183 This ratio may be 
lumped with other constants into a constant k. 

Finally, then 

F = ^W- ( 8 ), 

e r 

This flux must be made as small as possible in order 
to obtain maximum range. 

Bandwidth. One way to make F small is to de¬ 
crease A/. For carrier-wave [c-w] code reception it 
may be decreased until the tuned receiver circuit 
is set into self-oscillation. 

For voice reception, when A/ decreases, the S/N 
ratio required for intelligibility increases, causing 
an increase in the constant k. The rate of increase 
of k depends on the shape of the frequency-response 
curve and the frequency of peak response. The ex¬ 
perimental work on the type E system (see “Pre¬ 
amplifier” in Section 4.4.2) indicates that the values 
of /c\/A/ and of the flux F required for speech in¬ 
telligibility are a minimum for a peak response near 
1,500 cycles per second with a bandwidth (6 db 
down) of about 700 cycles per second. In later work 
on the aircraft systems (see “Amplifier” in Section 
4.4.3), it seems that better intelligibility may be 
obtained if this pass band is not sharply limited on 
the high-frequency side. The results of studies on 
pass bands and noise in sound-powered phones 41 
and similar devices should be applied to this prob¬ 
lem. 

Cell Area and Solid Angle of View. Another way 
to make F small is to decrease the effective cell 
area a. With a given optical system this involves a 
proportional decrease in the solid angle of view Q v . 
The angle Q v will be taken as bounded by the 
directions in which the receiver response falls to 
half the peak response [hpr]. 

If a given hpr receiver solid angle is specified, it 
may be obtained with a smaller and smaller cell 
area (and higher and higher sensitivity) by decreas¬ 
ing the focal length of the receiver lens or reflector 
up to a certain point. The limiting cell area is given 
by the relation 18 > 23 

<») 

In practice, as a result of optical aberrations, the 
minimum value of a is some 50 per cent greater than 
this value. 



104 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


This limit is reached with an //number of the 
optical system of about //0.5 for a one-surface de¬ 
tector like a phototube; //numbers down to about 
//0.1 may be used with two-surface detectors like 
thallous sulfide [TF] photoconductive cells (see 
Chapter 3), if they are mounted with their sensitive 
surfaces parallel to the axis of a reflector. The 
effective cell area a of a two-surface detector 
mounted thus is approximately the area of the 
double surface. Such a detector thus can have twice 
the solid angle of view of the same size one-surface 
cell for the same effective mirror area. 

Mirrors are required in order to attain either of 
these //numbers. With further decrease of the focal 
length below the values implied by these //numbers, 
the mirrors get deeper and deeper, and the average 
distance of the cell from the mirror surface in¬ 
creases. Then a must increase in order to keep Q„ 
constant, which causes the attainable receiver sensi¬ 
tivity to decrease again. 

For a maximum sensitivity, then, with a given A, 
the value of a required is given approximately by 
equation (9). 

The smaller the angle of view required, the higher 
the sensitivity of the detector cell which may be 
used. This parallels the relation for transmitters be¬ 
tween beam angle and intensity and leads again to 
the conclusion that a wide-angle system is a short- 
range system. 

However, it is less important to have a small solid 
angle at the receiver than at the transmitter, since 
the best receiver sensitivity is inversely proportional 
only to the square root of the solid angle of view, 
while the transmitter intensity is inversely propor¬ 
tional to the beam solid angle itself. 

Laboratory Methods. The directivity pattern of 
receiver response as a function of direction may be 
determined in the laboratory by placing the receiver 
on a rotating table to detect a weak source some 
distance away. The hpr angle of view may be found 
from this pattern. 

If the photodetector and associated circuits are 
linear, the ratio of the maximum response of the cell 
in the optical system in a uniformly illuminated 
field to the maximum response of the cell alone is 
called the receiver optics factor O r . In cells uni¬ 
formly sensitive over their surface, O r is equal to 
e r A/a', where a' is the projected area of the cell. A 
bare cell may be used for laboratory range measure¬ 
ments, and the range so obtained multiplied by 


VO r gives the range for the assembled receiver 
optical system. 

Backscatter. One practical limitation on receiver 
sensitivity in a two-way communication system is 
commonly the noise produced by radiation from the 
adjacent transmitter beam scattered back into the 
receiver by nearby objects or just by atmospheric 
haze. The backscatter from nearby objects may be 
avoided by proper location of the system. 

The backscatter from haze has been shown theo¬ 
retically 39 to be proportional to a 2 fi/d, where a is 
the hpi beam width, fi (^a) is the hpr receiver 
width, and d is the distance between centers of 
transmitter and receiver. It is not certain whether 
this analysis takes into account current ideas out¬ 
lined above concerning the relation of receiver'sensi- 
tivity to angle of view. But the general conclusion 
is certainly correct: the noise is large when the hpi 
and hpr angles are large, and also w'hen transmitter 
and receiver are close together. The latter is com¬ 
monly the case, the two units being placed together 
in a single transceiver head for convenience. 

Backscatter in wide-angle systems having a trans¬ 
ceiver head may make duplex operation impossible. 
In duplex, transmitter and receiver are continuously 
ready to operate, permitting great naturalness of 
conversation. But if the transmitter feeds back opti¬ 
cally into the receiver, it may drown out the distant 
station, and send-receive operation must then be 
used, with the transmitter and receiver energized 
alternately, as with a press-to-talk button. In this 
case, the transmitter line must be filtered from any 
modulation or ripple when the system is in the re¬ 
ceive condition. Even so, photocurrent noise from 
the remaining steady backscattered radiation may 
still be serious. If so, the transmitter intensity must 
be reduced or turned off altogether, either electri¬ 
cally or by a mechanical shutter, during reception, 
in order to achieve threshold sensitivity. 

Daylight Operation. The noise produced by steady 
backscatter is trivial compared with that produced 
in phototube and TF cell receivers by daylight. The 
operational ranges of NIR systems in daylight may 
consequently be much decreased from night ranges. 
(Ultraviolet systems are not so much affected, as 
they can be filtered to receive only waves shorter 
than 2,900 p where almost no sunlight comes through 
the ozone layer in the atmosphere.) 

In vacuum phototubes, the noise increases as the 
square root of the d-c current, which is proportional 



INTRODUCTION 


105 


to the total steady flux received. For a uniformly 
bright field of view, the flux is proportional to the 
product of and A. With gas-filled phototubes and 
TF cells, the expression for the change produced by 
background light is less simple. The TF cells are less 
affected than either kind of phototube, but with 
these cells there is the additional complication that 
the cell resistance is decreased by the illumination, 
necessitating a change in the load resistor if opti¬ 
mum performance is to be maintained (see “Pre¬ 
amplifier,Section 4.4.4, for a circuit which- may 
eliminate this complication). It seems that lead 
sulfide (PbS) photoconductive cells are very little 
affected by steady background light (Chapter 3). 

With detectors which are affected markedly by 
background radiation, very narrow-angle systems 
are much less affected by daylight than are wide- 
angle systems both because the solid angle of view 
is smaller and because, with Army (land) systems 
at any rate, a smaller fraction of sky is included in 
this solid angle. The Lichtsprechers (Section 4.3.1), 
which have hpr angles of % degree and PbS cells, 
seem to have no loss of range in daylight. The 
Japanese light-beam telephone mentioned in Section 
4.3.1 has an iris diaphragm over the receiver cell to 
give a large angle at night and a small angle in the 
daytime. 

This Japanese system and some other phototube 
systems with hpr widths less than 1 degree showed 
changes of about 20 per cent between the night and 
day vacuum ranges, 9 but the cells used had very 
high noise which would reduce the apparent size of 
the effect. The daylight vacuum range of the Signal 
Corps optiphone (Section 4.5.1), which has a similar 
receiver, seems to be less than the night range by a 
factor of about 3. In one test on TF-cell receivers 
with all-around view, background illumination 
equivalent to an overcast north sky caused a loss 
equivalent to a factor of 3 in vacuum range, even 
with proper load resistor adjustment (see “Pre¬ 
amplifier,” Section 4.4.4). 

Probably none of the systems to be described (ex¬ 
cept perhaps those using PbS cells) will work if 
sunlight falls on the receiver. An NIR filter and a 
sunshade over the receiver may materially improve 
daylight communication with the wide-angle sys¬ 
tems and use of a PbS cell may bring still greater 
improvement. More experimental work on these 
points is needed. 

Experiments with operation of a wide-angle TF- 


cell receiver in the presence of searchlights, star 
shells, and gun and shell flashes are reported under 
“Operational Tests” in Section 4.4.2. 

Revised Range Equation 

Combining equations (1), (6), (8), and (9), we 
have for the limiting vacuum range 

*•*=(!&)** 7T^=’ (10a) 

\kM I Q r ya 

or 



The change from k to k' and k", in equations (10b) 
and (10c) respectively, is introduced to take account 
both of the deviation of actual systems from the 
ideal limit case given by equation (9) and also of 
certain geometrical factors which depend on the 
shape of the detector cell. 

Equations (10b) and (10c) are generally the most 
useful in designing a communication system since 
they involve the solid angles explicitly, and these 
are usually among the first military characteristics 
specified. The last equation has the further advan¬ 
tage that it involves the detector cell area explicitly, 
and this quantity may be fixed by commercial 
availability for cells of a given type. 

The ranges R v are of a different order of magni¬ 
tude from the limiting ranges encountered, for ex¬ 
ample, in radio communication. Thus the maximum 
value of R v theoretically obtainable from a 100- 
watt source with beam widths of about 15 degrees, 
18-inch mirrors, and present detector cells is of the 
order of 50 miles. This corresponds to an ACW 
communication range of about 8 miles. While this 
short range is unsatisfactory for many military pur¬ 
poses, it does offer the advantage that the range of 
enemy detection, even with specially designed search 
receivers, is also limited to distances of the same 
order of magnitude. 

Auxiliary Equipment 

A number of auxiliary devices are commonly used 
with NIR communication systems, such as NIR 








106 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


image-forming devices (image tubes) for visual 
sighting on a distant source; stabilized platforms 
(stable tables) with NIR, HR, or far infrared 
[FIR], 8 to 13 p, training devices to maintain align¬ 
ment automatically especially with narrow-beam 
systems; and test or monitoring devices to check 
transmitter and receiver operation. 

Image Tubes. The types now used on equipment 
to be described include the charged-phosphor type 
AM metascopes and types C 3 and C 4 electron tele¬ 
scopes. These are described in the Summary Tech¬ 
nical Report of Division 16, Volume 4. 

Stabilized Platforms. Wide-angle systems, with 
beam angles over about 5 degrees, depending on the 
military use intended, may be mounted on gimbals 
and manually trained on the distant source with the 
help of an image tube. All-around systems, with 
beam angles over some 100 degrees and the resulting 
great reduction in range and security, are used only 
where manual guiding is very undesirable and where 
a transceiver is to be fixed in position on a moving 
craft or vehicle. 

Narrow-angle systems, with beams less than 5 
degrees, may be used from moving stations only if 
the systems are automatically guided. They may be 
guided by focusing radiation from the distant sta¬ 
tion on a split detector cell and applying the ampli¬ 
fied differential signal from opposite halves or 
quadrants of the cell to a driving motor. This motor 
then rotates the apparatus so as to keep the distant 
station centered in the field of view. The cell may 
respond to an NIR source or beacon or it may re¬ 
spond in the IIR or FIR to the naturally emitted 
heat radiation from the other station (provided it 
is a ship or a heated vehicle). 

If the NIR is used the cell might be the detector 
cell of the communication system, though cells and 
circuits for such an arrangement have not been 
worked out; the distant source might be the com¬ 
munication source, although an all-around beacon 
is better as the systems are then not so troublesome 
to line up initially. 

For Navy use, where the only variable coordinate 
of the distant ship station is its azimuth, tracking is 
simplified by mounting the system on a gyroscopi- 
cally stabilized horizontal platform; then only a 
two-element tracking cell is needed. The transceiver 
units must be very light in weight to be used with 
present platforms. In experimental BuShips tests 
successful tracking from such platforms to de¬ 


gree of arc® has been obtained with FIR systems 
trained on small ships at distances up to 4 miles in 
average weather. 

Test Devices: Microbeacon. Source monitoring 
devices are necessary for checking proper transmit¬ 
ter operation in some of the polarization systems, 
but for most voice systems to be described the oper¬ 
ation is adequately monitored by simple visual or 
image-tube observations without any additional 
apparatus. 

Similarly, detector cell and receiver operation is 
usually checked simply by listening for the charac¬ 
teristic hiss of cell noise in earphones or loudspeaker 
and noting the increase in noise produced by placing 
a light, a match, or a cigarette in the field of view. 
For more guarded and accurate field testing, two 
portable microbeacon test sources have been con¬ 
structed by University of Michigan Contract 
NDCrc-185. 40 

The first microbeacon, shown in Figure 4, is oper¬ 
ated from a 110-volt 60-cycle per second a-c ship 
supply. It consists of a small tungsten lamp whose 
light is modulated by a sector disk used as a me¬ 
chanical chopper at 90 or 1,500 cycles per second 
as desired. A virtual image of the source is formed 
by the polished surface of a steel ball, the intensity 
being thus diminished as discussed in “Laboratory 
Methods,” in Section 4.1.3. The light from the image 
passes out of the box through an aperture covered 
by a suitable NIR filter. A variable resistor in series 
with the tungsten lamp has a dial calibrated to read 
the emergent NIR flux in mile-holocandles (see 
Appendix). 

In operation the beacon is held at some standard 
distance, such as 10 feet, from the receiver to be 
tested. It is pointed at the receiver and the lamp 
resistor is adjusted until the code tone can barely 
be detected above the noise. The receiver is pro¬ 
nounced satisfactory or unsatisfactory according 
as the emergent flux from the beacon is then below 
or above some specified maximum allowable 
value. 

The second microbeacon 40 is an ingenious, con¬ 
stant, and simple device, which was constructed for 
checking the operation of the plane-to-plane recog¬ 
nition system (Section 5.3),' but which would be 
equally applicable, with appropriate frequency 
changes, to any other NIR receiver. A ^4-watt neon 

0 Information supplied by courtesy of Section 660E, 
Bureau of Ships. 








PRE-NDRC SYSTEMS: GENERAL DISCUSSION 


107 



Figure 4. Microbeacon test source, with cover removed. 


lamp acts both as a 90-cycle per second relaxation 
oscillator and as a source of light. Two other neon 
lamps serve as voltage regulators to compensate for 
the aging of the 200-volt batteries, and one of these 
lamps serves as an indicator of battery deteriora¬ 
tion. The unit, including batteries, is in a box 
3x4i/2xlOM> inches in size, and may be held in one 
hand in front of the receiver to be tested. The inten¬ 
sity and frequency of the lamp are almost inde¬ 
pendent of temperature or of the age of the bat¬ 
tery. 

One of the code systems to be described in Chap¬ 
ter 5 also has a build-in microflux lamp for testing 
receiver operation. Such a method could be easily 
adapted for use in the voice systems discussed in 
this chapter, if desired. 


42 PRE-NDRC SYSTEMS; GENERAL 
DISCUSSION 

The success of optical voice communication sys¬ 
tems has been dependent on the sources and sensi¬ 
tive detector cells available (that is, past the lab¬ 
oratory experimental stage) at any given time. As 
for sources, voice modulation of carbon arcs, man- 
ometric flames, and probably filament lamps was 
used before the turn of the century. Successful vi- 
brating-mirror and vibrating ribbon-shutter systems 
for speech frequencies were produced in the sound 
motion-picture research during the 1920’s. Efficient 
modulable gaseous discharge sources of infrared 
radiation became available only in the last decade. 

Selenium photoconductive cells were the only in- 




108 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


frared audio receivers in the early years of the cen¬ 
tury. The Case Thalofide cell (Section 3.3.1) ap¬ 
peared during World War I. Cesium-surface photo¬ 
tubes were first produced commercially in the early 
1920’s. Lead sulfide cells and other HR detectors 
are the products of the last ten years. 

4,2,1 Early German Work 

A bibliography and discussion of some early op¬ 
tical voice communication systems have been given 
by Thirring. 1 As early as 1901, Simon in Gottingen 
obtained ranges up to 1 km with a 3-foot search¬ 
light having a voice-modulated arc, and much 
greater ranges were claimed by Riihmer in 1904. 
The first successful systems from a military point of 
view were developed by Simon (Germany) and 
Thirring (Austria) working in cooperation with the 
Siemans-Halske Company in 1917. 

The Thirring system was put into production but 
did not reach the front before World War I was 
over. The usual source was a 14-inch searchlight, 
with the arc modulated by inductive coupling be¬ 
tween its circuit and the circuit of a carbon micro¬ 
phone. Glow discharge lamps, 5-watt Nachtlampen , 
and others were also used successfully in such a 
reflector but gave shorter ranges. The receiver em¬ 
ployed a 1-millimeter selenium photoconductive cell 
on which the electrodes were placed in a grid ar¬ 
rangement. This was set at the focus of a 22-centi¬ 
meter diameter, 30-centimeter focal length lens. It 
was connected to headphones through a four-stage 
triode amplifier. The receiver sensitivity was limited 
only by cell noise, which was high; the variation of 
cell noise with cell area is discussed in Thirring’s 
paper. 

The system is reported to have had an ACW voice 
range of nearly 8 kilometers (5 miles) and a much 
longer range with a buzzer code tone which was 
used for a call signal. The angles were probably 
about 1 degree for the transmitter and % degree for 
the receiver. The system apparently was designed 
to be portable by two men and to operate from a 
gasoline generator. 

No filter is mentioned in the account. Perhaps the 
beam was narrow enough for security without it. A 
weak infrared filter could have been used as sele¬ 
nium cells can be constructed to respond near to 
1 n. 

It is likely that this system was the predecessor 


of the Lichtsprecher systems to be described later 
which were produced for the German Army some¬ 
time before 1934 and which figured in World War II. 
The Zeiss Works were associated with both systems 
and there are a number of common features, for 
example, in the receiver design. 

An interest in very narrow beams (under 1 de¬ 
gree) characterized the early German studies. This, 
together with the static character of the last war 
which made possible prepared and stationary posi¬ 
tions for transceivers, guided subsequent work there 
and elsewhere almost exclusively into narrow-beam 
systems up until the studies reported here. A narrow 
beam makes for great security, but such systems 
easily get out of alignment and are hard to line up 
again in field use, hence they are quite unsuited to 
mobile warfare. This may be one of the reasons 
why the Lichtsprechers and the Japanese narrow- 
beam systems were of so little military importance 
in World War II even though they were commer¬ 
cially produced for many years and although such 
secret, wireless, voice communication devices were 
greatly desired for military purposes even in World 
War I. 

Several ingenious German visual blinker code 
devices of higher security were evolved during 
World War I which were based on polarization- 
modulation and spectral-modulation of the light 
beam. 2 One of these may have reached the produc¬ 
tion stage. 

4,22 American Systems in World War I 

Invisible ultraviolet and infrared blinker systems 
saw field use in World War I in guiding airplanes to 
landing fields and in keeping convoys together. 4 The 
ultraviolet sources were observed with telescopes 
having a fluorescent screen in the focal plane. The 
infrared sources in the convoy system were detected 
by a receiver using the Case Thalofide cell (Chap¬ 
ter 3). Receivers converting the infrared for visual 
blinker observations were, of course, not possible 
then nor for another 20 years until after the devel¬ 
opment of phosphors and infrared photoemissive 
cells and great advances in electron optics. 

Case Code System 

An infrared system apparently similar to this con¬ 
voy system was demonstrated by the Case Research 
Laboratory in October 1917 to representatives of 



PRE-NDRC SYSTEMS: GENERAL DISCUSSION 


109 


the Army and Navy. 3 In this demonstration the 
Thalofide cell was mounted in a 24-inch reflector 
and was part of a triode oscillator circuit connected 
to earphones. Reception of a signal by the cell 
caused a change in its resistance and a change in the 
pitch of the audio note in the phones. The source 
was a 60-inch Sperry searchlight covered by a shut¬ 
ter and filter. The filter was made by combining 
Wratten filters 91, 45, and 53 into a single unit called 
Wratten 740, which was sealed at the edges against 
moisture. This filter has a cutoff near 0.8 p, and 
transmits 50 per cent at 1 p. 

With this system, infrared blinker signals over 
a range of 18 miles from Fort Hancock to the Wool- 
worth Building caused very distinct changes of pitch 
in average weather. Further tests were carried out 
in February 1918 with the Coast Artillery at Fort 
Monroe, Virginia. 

At the end of World War I smaller versions of 
this system had been built for two-way communica¬ 
tion. In these, the receiver and transmitter both had 
8-inch mirrors and were incorporated in a single 
transceiver head. The source was an 8-volt signal 
lamp and the beam was modulated by a butterfly 
shutter. The system operated from a storage battery, 
had a range of 4 miles, and could be carried by 
two men. 

Case Voice System 

Two voice systems were also constructed. In these 
the source was an acetylene flame, modulated by 
speaking into a horn connected to the base of the 
burner. This source was about % square inch in area 
and had an intensity of about 150 candlepower. The 
receiver was the Thalofide cell feeding into a three- 
stage amplifier. 

When the source and cell were each used in 24- 
inch mirrors, the range was 5 miles on a clear night. 
Beam angles were probably of the order of 4 degrees. 

With 12-inch mirrors, a 2-mile range was obtained, 
probably with beam angles of about 8 degrees. 

This work was not carried further after the con¬ 
clusion of World War I. 

Later Work 

American laboratories also studied voice-modu¬ 
lated carbon-arc systems, but apparently without 
any results of military value. 

Later work was carried out principally in the 


development of sound motion pictures. These have 
all the elements of optical communication, plus a 
troublesome intermediate stage of recording on film. 
Much study was devoted to sources, methods of 
modulation, and detectors, though with emphasis on 
ultraviolet rather than infrared radiation. Descrip¬ 
tions of this work may be found in the technical 
journals of the period, such as the publications of 
The Society of Motion Picture Engineers. 

The elements produced by this research were used 
from time to time in short-range demonstration ex¬ 
hibits to amaze the curious by “talking on a beam 
of light.” Similar short-range systems, such as QST, 
have been presented as playthings in radio amateur 
magazines, but appear not to have been intensively 
developed further in this country for military pur¬ 
poses until the start of World War II. 

423 Existing Systems at Beginning of 
NDRC Studies 

At the time NDRC began work on optical com¬ 
munication systems, the systems now known to 
have been in operation were the German and Japa¬ 
nese vibrating-mirror systems and an Italian sys¬ 
tem with a modulated tungsten lamp (see Sections 
4.2.4 and 4.3.1). The German units used TF-cell and 
PbS-cell receivers, having changed from the selenium 
receivers of the Thirring system after Case’s dis¬ 
covery of the thallous sulfide cell. The other sys¬ 
tems employed phototubes. 

Another German system which reached the stage 
of an experimental field unit during World War II 
was apparently an r-f FM system using a cadmium 
compound for a detector cell and perhaps an infra¬ 
red mercury lamp source (Section 4.5.1). 

The British at that time were working on several 
tungsten lamp systems, all with phototube receivers 
(Section 4.2.4). 

In this country, experimental models of the fol¬ 
lowing systems were being developed: the Signal 
Corps optiphone (Section 4.5.1), using supersonic 
diffraction; the Western Union aural signal unit 
(Section 4.4.1) using a modulated arc; and the RCA 
type R-2 (Section 4.3.1) with a vibrating mirror. 
Studies were also being carried out with a Kerr cell 
polarization system (Section 4.6.2). All these sys¬ 
tems had phototube receivers. Both the foreign and 
American systems had narrow-angle transmitters 
with beams 1 degree wide or less; most of them had 




110 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


narrow-angle receivers suitable for daylight opera¬ 
tion. 

This information is based only on accessible re¬ 
ports and is necessarily incomplete. 

4 2 4 Modulated Filament Lamp Systems 

Systems using a modulated tungsten filament will 
be mentioned here briefly for reference, since this is 
an important type of modulation device even though 
it was not used in any of the voice systems devel¬ 
oped by NDRC. Two code systems of this type, the 
type D-2 recognition system and the plane-to-plane 
recognition system will be described in Sections 5.2 
and 5.3, respectively. 

The advantage of such systems is that the source 
arrangements are small and uncomplicated. Like the 
modulated arc lamps they have no moving parts, 
and the lamps are commercially available. The dis¬ 
advantage is that the modulation ratio depends on 
the heating and cooling time of the filaments. For 
voice modulation they must, therefore, be made of 
very fine wire; but even when they are so fine as 
to be fragile, the ratio of modulated to total light 
is not very large at the speech frequencies over 
1,000 cycles per second which are important for 
intelligibility. Obtainable ranges with such voice 
systems are therefore small. Another disadvantage 
for voice use is that the light output is a very non¬ 
linear function of either current or voltage. These 
features are drawbacks only with voice modulation; 
for code operation, at low frequencies, modulated 
tungsten lamps are very good sources and give good 
communication ranges. 

Italian Photophones 

Two Italian voice systems, the type F.F.115 pho¬ 
tophone and another similar system using concave 
mirrors instead of lenses in the optical system, make 
use of modulated tungsten filaments of about 2-watt 
rating. The beam angles are about 0.5 degree. 

British Systems 

Several British narrow-angle systems are similar 
to the Italian ones. All have cesium phototube re¬ 
ceivers. None has over about 1-mile range in day¬ 
light; perhaps the night ranges are two or three 
times as great. 

A British wide-angle system, the ASE photo¬ 
phone, using a modulated 2-volt 1-watt tungsten 


source with a 10-degree transmitter beam, was de¬ 
signed for communication at night between moving 
infantry groups at distances up to 200 yards. A 360- 
degree all-around response was obtained by using 
a semicylindrical S/T thallous sulfide cell. The 
transceiver unit weighed about 25 pounds. This sys¬ 
tem may be compared with type W (Section 4.3.2) 
which might be used for the same purpose. The lat¬ 
ter has about the same weight and size but much 
greater range because of its higher transmitter 
power and smaller receiving angle. The advantage 
of the larger receiver angle in the ASE system is 
that the unit is continuously ready to receive from 
any direction without a prearranged schedule. 
Whether this advantage is worth the sacrifice in 
range is for the Armed Services to decide, but it 
would seem that modification of the ASE system 
in the direction of type W would make it much 
more useful. 

43 VIBRATING-MIRROR SYSTEMS 

Audio-frequency mechanical modulation is now 
the most common method used in near infrared mil¬ 
itary communication devices. The simplest mechani¬ 
cal method is the use of a rotating “chopping disk” 
(type D system), but this is capable of only a lim¬ 
ited kind of code communication and will be de¬ 
scribed in Section 5.2. 

The other mechanical voice-modulation devices 
are the vibrating ribbon-shutter, used extensively 
in sound motion pictures, and the vibrating mirror. 
Only the latter has been used in NIR military com¬ 
munication systems. 

In the basic vibrating-mirror arrangement, the 
light is split by a grid into bands which are reflected 
from an oscillating mirror onto another (identical) 
grid. The bands move back and forth across this 
second grid with the motion of the mirror and are 
thus transmitted by the second grid with variable 
intensity corresponding to the voice modulation. 

4 3,1 Other Foreign and American Systems 
Type R-2 

In the simplest arrangement, used in the RCA 
type R-2 unit, 10 the grids are opaque, and are used 
with a lens system. This system has a 1-degree hpi 
width and a 3-degree hpr width. The detecting cell 
is an RCA 921 phototube. The equipment is portable 



VIBRATING-MIRROR SYSTEMS 


111 


and weighs about 49 pounds. It has a night infra¬ 
red ACW voice range of about 1.6 miles. The excel¬ 
lent mirror galvanometer is based on the one used 
in RCA sound-recording systems and has a flat 
response from 200 to 3,000 cycles per second. 

Type G 

The R-2 system was used as the basis for the 
design of the Navy type G system by the same com¬ 
pany. The latter is a narrow-beam system of higher 
power to be mounted on a stabilized platform with 
FIR training system for ship-to-ship communication 
at distances up to about 4 miles. It was thought that 
this might replace the type E ship system (Section 
4.4.2), giving greater security than type E because 
of the narrow beam. The engineers working under 
Contract OEMsr-990 to develop the latter system 
were consulted in the development of type G, and 
recommended the replacement of the receiver photo¬ 
tube by a TF cell (for reasons listed in discussing 
“Photodetector Cells ,, in Section 4.4.2) and supplied 
information on receiver circuits and on conditions 
for the use of such cells. d 

Japanese Light-Beam Telephone 

Other vibrating-mirror systems involve more 
sophisticated variants of this grid design. In the 
Japanese light-beam telephone, 9 the opaque strips 
of the second grid are aluminized on one side so that 
they reflect the incoming light to the receiver. 
(Type W, to be described in Section 4.3.2, employs 
a much more ingenious reflecting grid.) Transmitter 
and receiver thus use the same optical aperture, and 
the design is made very compact. The receiver em¬ 
ploys a phototube. The hpi and hpr angles are 0.1 
degree and about 1 degree, respectively, and the 
night infrared ACW voice range is about 1.3 miles. 
The weight of the field equipment is 110 pounds 
exclusive of power supply. 

German Lichtsprechers 

Three German vibrating-mirror systems were in 
field use in World War II. They are the Licht- 
sprecher (abbreviated “Li”) 60/50, the Li 80, and 
the Li 250/130, all made by Zeiss. 5 ’ 6 ’ 7 The numbers 

d A PbS cell is now being considered for daylight operation 
of this system. The system was to have been tested at 
BuShips test station on November 26, 1945, but due to the 
decommissioning of the USS Marnell these tests were de¬ 
layed. This information is supplied by courtesy of Section 
660E, BuShips. 


refer to the transmitter-receiver apertures in milli¬ 
meters. The field equipments weigh 30, 54, and 140 
pounds, respectively. All have narrow beams, down 
to % degree; the ACW day or night voice range of 
the largest is probably over 10 miles. The small 
effect of daylight and the very great ranges prob¬ 
ably result from the insensitivity to background 
light of the PbS receiver cells used in these systems 
for the last five years, and from the extension of the 
spectral response of these cells into the HR where 
atmospheric attenuation is less serious (see Section 
4.8). 


* 




Figure 5. Modulation system in Lichtsprecher 80 
(Zeiss). 


These three systems are marked by great elegance 
of design and by optical precision and sturdiness of 
construction. The transmitter arrangement used in 
the earliest, the Li 80, designed before 1934, is so 
unusual that a diagram is shown in Figure 5. Al¬ 
though it uses a vibrating mirror, it works on quite 
a different optical principle from that described 
above. There are no grids. At a point where the 
radiation beam is undergoing total internal reflec¬ 
tion in a large prism it is modulated by the mechan¬ 
ical oscillation of a glass surface (the small prism 
on the armature in the diagram) in the air behind 
the reflecting surface! The two surfaces are about 
y 7 of a wavelength apart (0.15 \i ); the effect results 
from the penetration of the light energy across this 
gap even at an angle of total reflection. More or less 
of the light passes into the second glass and so out 
of the totally reflected beam, according as the second 
surface is nearer or farther away. 

The other two Lichtsprechers are more normal 











112 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


vibrating-mirror galvanometer systems, although 
great ingenuity is again shown in the grid design. 
The grid lines are not opaque but are thin glass 
prisms so that instead of an average grid trans¬ 
mission of 25 per cent, as in the simplest vibrating- 
mirror system outlined above, the transmission is 
always 100 per cent (except for reflection losses). 
The modulation is accomplished by the diversion of 
more or less light out of the central beam into two 
side beams as the mirror vibrates and changes the 
fraction passing through different prism combina¬ 
tions. The effective width of the beam is thus tripled 
and the central intensity is doubled over that obtain¬ 
able if the grids were opaque. 

4.3.2 Portable, Hand-Held, Infrared 
Optical Telephone, Type W 

Description and Performance 

A major simplification has been achieved in an 
American vibrating-mirror system which conserves 
light by replacing all lenses by large-aperture con¬ 
cave mirrors and which eliminates the second grid 
by placing one grid on a parabolic surface and 
using it twice, once in transmission and once in re¬ 
flection. This is type W, employing the so-called 
“reflex-image optical modulation system” which was 
developed under NDRC. 12 

Course of Development. The original develop¬ 
ment, initiated by Section 16.5 under Contract 
OEMsr-1073 with the University of California in 
1943, led to a reflex-image system capable of voice- 
modulating the radiation (nonvisible ultraviolet) 
from a carbon arc drawing several kilowatts. When 
it was realized that the same principle would lend 
itself to a small lightweight unit as well, two port¬ 
able hand-held instruments were designed and con¬ 
structed. 

Tungsten-filament lamps, (infrared) were used as 
a matter of convenience in early tests. The results 
proved so promising that it was decided by agree¬ 
ment between Sections 16.5 and 16.4 to place major 
emphasis on the further practical development of 
such a portable system utilizing NIR radiation. 

This development was carried out under Project 
Control AC-226.03 at the request of the Army 
Air Forces to provide the ground unit of an infrared 
plane-to-ground communication system with a de¬ 
sired ACW range of 3 miles, and under Project Con¬ 


trol NS—371, at the request of the Bureau of Ships, 
to provide a system for two-way, day or night com¬ 
munication between amphibious craft at ranges up 
to 2 miles. 

The electrically modulated plane unit (P-G sys¬ 
tem) of the plane-to-ground system was already 
under development by Northwestern University un¬ 
der Contract OEMsr-990 and will be described, 
together with the relationship of the two units, in 
Section 4.4.3. Type W may also be used for the 
plane unit of the plane-to-ground system. 

It was requested that the ground unit have an 
NVR of 50 feet, weigh less than 40 pounds so that 
it could be carried to earth on the person of a para¬ 
trooper, have a self-contained power supply with an 
operating life of at least 15 minutes, be subsequently 
operable from a plane or car storage battery, and 
have hpi and hpr angles of 8 ± 2 degrees and 10 
degrees, respectively (see Section 4.4.3). 

Five separate units were constructed during the 
infrared type W development, of which the last two 
have the highest power (100 watts) and appear to 
work most successfully. Since the modulation 
method is independent of the source used, the system 
may be employed for either the UV or NIR and can 
probably be adapted to take interchangeable sources 
and cells for these regions. It can also be used in the 
visible and has the best transmitter so far developed 
for the HR (see Section 4.8). 

Only the infrared type W system is described in 
this chapter, since the ultraviolet work of the Uni¬ 
versity of California is reported in Chapter 6 of the 
final report, Section 16.5. (See STR of Division 16, 
Volume 4.) This performance of two similar type W 
systems communicating to each other will be dis¬ 
cussed in the present section. Their use as ground 
units to communicate with cesium lamp airborne 
units in the P-G system is described in Section 4.4.3. 
Much of the operational data on type W perform¬ 
ance was obtained from the tests reported therein 
the third unit constructed. The termination of the 
war prevented any extensive tests under NDRC on 
the two final units. 

General Design. An assembled and a disassem¬ 
bled transceiver of the final model are shown in 
Figure 6. The filter shown over the receiver in Fig¬ 
ures 6 and 8 is not necessary for night operation 
and could be replaced by clear glass. 

Each transceiver contains (1) a mechanical- 
optical reflex modulator, (2) a photodetector and 





VIBRATING-MIRROR SYSTEMS 


113 



Figure 6. Assembled and disassembled type W transceiver. 


receiver mirror, (3) a combined transmitter and 
receiver amplifier, (4) a microphone and earphone, 
and (5) an infrared metascope for sighting. 

The transmitter, the optical components of which 
are sketched in Figure 7, modulates the steady 



radiation emitted by a special 100-watt, tungsten 
filament lamp operated at a color temperature of 
about 3400 K. The radiation is focused by a gold- 


plated ellipsoidal mirror, 4.5 inches in diameter, 
onto an electrically driven concave vibrating mirror, 
%6 x %-i n ch oval, radius of curvature 5% inches, 
after passing through a concave parabolic grid mir¬ 
ror, 4.25 inches in diameter, which has its focus at 
the surface of the small vibrating mirror. The grid 
mirror, each side of which has a radius of curvature 
of 11% inches, is coated with reflecting strips of 
gold, approximately % 6 inch wide, alternated with 
clear spaces of exactly equal width. The vibrating 
mirror focuses an image of the grid back on the sur¬ 
face of the grid mirror, and the image of the fila¬ 
ment on the vibrating mirror acts as a source of 
variable intensity. The grid mirror collimates this 
radiation into a 4x5-degree beam in which the vari¬ 
ations in intensity are produced through the changes 
in the amount of radiation reflected from the grid 
mirror as the image of the grid is vibrated back and 
forth across its surface. A Lucite beam-spreader 
plate containing a number of little lenses may be 












114 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


used to enlarge the transmitted beam to 10x12 de¬ 
grees if desired. The modulator power required is 
about 0.1 watt. 

The receiver uses a type A TF cell (Chapter 3) 
in a 4%-inch diameter, 3-inch focal-length mirror to 
give a field of view of about 10x10 degrees. It has a 
four-stage amplifier which may be operated from a 
vibrator power pack or preferably from a light¬ 
weight 90-volt B battery. 



Figure 8. Carrying and operating type W. 


The transceiver head is about 5x10x12 inches in 
size and has a carbon microphone and an earphone 
built into its left side so that it may be held on 
the right shoulder in communication position and 
guided by one hand, as shown in Figure 8. For sight¬ 
ing on the distant station, a type AM phosphor 
metascope viewing tube is attached, though it 


appears that the somewhat heavier type C 4 infrared 
electron telescope will be required for sighting at 
the ultimate ranges. Arrangements are provided for 
the use of noise-canceling lip microphones and 
helmet headsets in noisy locations or for remote 
operation from an auxiliary phone on a long exten¬ 
sion if desired. 

One complete transceiver weighs about 8 pounds. 
The power pack containing a 6-volt, 25-ampere-hour 
storage battery and a small 90-volt B battery, 
weighs approximately 10 pounds (several pounds 
more if an electron telescope is used) and is carried 
by a strap over the shoulder. This supply will oper¬ 
ate the 100-watt lamp for about 1 hour. 

The units are reasonably rugged although the 
rather critical vibrating-mirror setting in the labora¬ 
tory models is not so stable as it should be for field 
operation. The units can be made fairly watertight. 
No high-precision optical parts are used and all 
radio parts are standard production types. 

Security. An Ohio State University [OSU] filter 
(Chapter 2) or a Polaroid XR3X-type filter with a 
K p 850 value (see Chapter 2) of about 0.30 may be 
used over the transmitter. The NVR appears to be 
about 2 per cent of the range, or about 100 yards for 
the 5-degree beam and 50 yards for the 10-degree 
beam. 

The maximum ACW range of detection of the 
type W transmitter by a type C 4 image tube is esti¬ 
mated, from image tube experience with other 
transmitters, to be of the order of 6 miles. 

Range. The final model type W systems have not 
been field-tested, but the ranges from one to the 
other of them may be computed from laboratory 
data and from measurements on other similar sys¬ 
tems. An ACW night range of 3 miles is expected 
with the 4x5-degree beam, and of about U /2 miles 
with the 10xl2-degree beam. Expected ranges to the 
P-G unit are given in Section 4.4.3; to type E, under 
the heading “Mixed Use” in Section 4.1.2. 

Evaluation. The type W system is simple, versa¬ 
tile, and cleverly designed and has approximately 
the performance requested for it. If necessary the 
NVR can be brought down to the 50 feet requested 
in Project Control AC-226.03 by a suitable filter, 
probably with little loss in communication range. 
It, or some closely related system, ought to be con¬ 
sidered for use as a general supplement to handy- 
talkie and walkie-talkie radio sets in close military 
operations. 










VIBRATING-MIRROR SYSTEMS 


115 


Transmitter 

Construction of Elements. Ordinary 40-watt bulbs 
were used in the first three units. For the last two 
models special 100-watt bulbs were made by the 
General Electric Company. A burned-out lamp can 
be replaced in 1 minute. 

The magnification of the ellipsoidal mirror varies 
from 5X at the center to 3X at the edge, so that the 
surface of the vibrating mirror is rather uniformly 
covered by the diffuse image of the filament. None 
of the mirrors requires great accuracy of shape. 

The grid was constructed on the parabolic glass 
plate by evaporation of gold through a metal pattern 
until an opaque layer was obtained. The width of 
the lines is y 16 inch, which is small enough to permit 
full modulation by the mirror galvanometer, but not 
so small as to impose stringent optical conditions 
on the concave vibrating mirror which forms the 
grid images. 

The vibrating mirror is concave spherical so that 
it forms sharp images of the grid lines by reflection 
-back on the parabolic grid surface. The image strips 
should overlap the actual strips by one-half their 
width when the galvanometer mirror is at rest. As 
the latter mirror vibrates, more light is reflected 
from the parabolic strips when it swings to one side 
and less when it swings to the other. For maximum 
modulation the image strips have an amplitude (dis¬ 
placement from rest position) of one-half their 
width. Overmodulation produces great distortion. 

In these models strong overmodulation or mechan¬ 
ical shock may produce permanent displacement of 
the vibrating mirror from its rest position. To re¬ 
store intelligibility, the parabolic mirror can be dis¬ 
placed laterally by an external eccentric pin until 
the grids and their images are again in the correct 
relation. A viewing port permits observation of them 
during this operation. 

Vibrating-Mirror Element. The vibrating-mirror 
element finally adopted was modeled after the one 
developed and used by RCA in its moving-picture 
sound-recording system. A schematic diagram is 
given in Figure 9. A soft iron reed R vibrates back 
and forth in the field of a permanent Alnico magnet 
P, as the reed is magnetized in one direction or the 
other by the current in the voice coil C. The reed is 
0.020 inch thick and %x 11 / 1Q inch in area. It is 
clamped at the bottom between German silver 
blocks and clears the outer pole pieces in the neutral 


position by 0.005 inch. The end of the reed presses 
the mirror block M against a 0.002-inch thick 
phosphor-bronze tape T, and motion of the reed 
rocks the block back and forth on this tape. The 
concave mirror has a radius of curvature of 5% 
inches and is of oval shape, % 6 x% inch in area. 
The voice coil impedance is about 100 ohms at 1,000 
cycles. 



Figure 9. Type W vibrating-mirror galvanometer 

element. 

Amplifier. The transmitter-amplifier is composed 
of the last two stages of the four-tube receiver- 
amplifier, Figure 10. A press-to-talk button throws 
a relay connecting the microphone transformer to 
the grid of the third tube and switching the output 
transformer from the earphones to the vibrating- 
mirror element. 

A low-pass filter network flattens out the response 
curve of the vibrating-mirror unit, which peaks at 
3,000 cycles per second. Low transmitter frequencies 
are cut out by a coupling condenser between the two 
tubes. The result is an overall transmitter and re¬ 
ceiver response almost constant from 300 to 2,600 
cycles. A volume control between the two tubes con¬ 
trols the transmitter modulation and is accessible 
by opening the case. 

Receiver 

Commercial Cetron tubes and photomultiplier 
tubes were tried at first. The latter were less suit¬ 
able because of the weight of the required 1,000- 
volt supply. Maximum ranges of about 1 mile were 
obtained with the Cetron tubes in the first two 
units constructed, but this range was greatly re¬ 
duced because of leakage across the tube if the 


I r FT) 






















116 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


humidity was high. When the change to TF cells 
was made, on recommendation of the NDRC con¬ 
tractor, these difficulties were resolved and a gain 
in sensitivity of 20 to 30 decibels was realized, in¬ 
creasing the ACW range of the same units to about 
3 miles. 

The TF cell is placed beyond the focus of the 
mirror so that the light from a distant source is 
spread on a disk % inch in diameter. This smooths 
the irregularities which would be present in the 
directivity pattern if the light were focused to a 
sharp image on the grid structure of the cell. 

Amplifier. The receiver-amplifier (Figure 10) is 
a conventional four-tube high-gain audio amplifier, 
containing some compensation for the decrease in 
TF-cell response at high frequencies. 

Cathode bias is used on the tubes to simplify re¬ 
placement. Between the first two tubes and the last 
two tubes is a decoupling filter in the plate and 
screen supply. Wire-wound resistors are recom¬ 
mended for low noise in the phototube and in the 
grid circuit of the first tube. 

The receiver volume control follows the second 


stage of amplification and is cut out when the 
push-to-talk relay is in the send position. 

The receiver draws about 0.7 ampere from the 
6-volt storage battery and 5 milliamperes from the 
90-volt B battery. The latter consists of cells from 
commercial batteries with a total weight of about 
15 ounces. Use of cells from a new kind of battery 
now available would reduce the weight to 6 ounces. 
Vibrator power packs constructed for early tests 
weighed 30 ounces. Such packs are noisy, reducing 
ultimate receiver sensitivity, but they make the 
equipment independent of dry-battery replacement 
in field use. 

Operational Tests 

During the type W development many range tests 
were made on Berkeley Pier, 3.5 miles long, in San 
Francisco Bay. Field tests of the two first units, 
which employed 40-watt lamps (2x3-degree trans¬ 
mitted beam) and phototube detectors, were also 
made at night at the BuShips test station at Fort 
Miles, Cape Henlopen, Delaware, in March 1945. 
One unit was on the USS Mamell , the other on a 



Ui 

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3 

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Ol 


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tr 

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(£) 


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TERMINAL STRIP 


Figure 10. Type W transmitter-receiver amplifier. 




























































ELECTRICALLY MODULATED ARC SYSTEMS 


117 


motor launch. The initial limiting range was 2,600 
yards, but this decreased steadily as phototube leak¬ 
age developed because of high humidity. 

After installation of TF-cell receivers in these 
units and construction of a third and improved unit, 
tests on Berkeley Pier gave night operational ranges 
of 2 to 3 miles with the 2x3-degree transmitted beam 
angle. One plane-to-ground test between two of 
these units in the summer of 1945 at Mills Field, 
Army Air Base, San Francisco, was unsatisfactory 
because of heavy airport traffic and unusually rough 
air and probably also because the beam angle used 
was only 5 degrees, too narrow for maintaining easy 
alignment with a hand-held unit. The noise-cancel¬ 
ing lip microphone used was successful in eliminat¬ 
ing transmission of background motor noise. The 
large effect of the backscattered radiation from the 
transmitter on this test dictated the installation of 
a press-to-talk button on the next two units con¬ 
structed. 

Because of the termination of World War II no 
further field tests have as yet been made for com¬ 
munication between naval craft. 

Operational tests from the third type W unit to 
the P-G system are reported in “Operational Tests” 
in Section 4.4.3. 

The ranges previously given above under “Range” 
for the fourth and fifth units built were computed 
from laboratory measurements. 

. Present Status 

One of the last three units constructed has been 
allocated to the Special Projects Laboratory at 
Wright Field for plane-to-ground tests. The first of 
these tests is reported in Section 4.4.3. It is expected 
that these tests and any attendant modifications of 
the units will be carried out in consultation with 
Northwestern University workers under Navy Con¬ 
tract NObs-28373 which is continuing the work 
begun under Contract OEMsr-990. The other two 
of the last three units are to be allocated to the 
Navy for ship-to-ship, ship-to-shore, and finally 
plane-to-plane tests. 

It is assumed that if these tests are up to expecta¬ 
tions, either the Navy or the Air Forces may insti¬ 
tute some commercial production program. 

Recommendations 

The system appears to have the military charac¬ 
teristics and performance requested for it as nearly 


as may be obtained in a laboratory model. The in¬ 
stability of the vibrating mirror constitutes a diffi¬ 
culty in the use of the present units. Whether still 
more source power is needed to meet the specifica¬ 
tions of Project Control AC-226.03 can be deter¬ 
mined only after further operational tests. 

It is not certain that the best combination of 
source, operating temperature, and infrared-trans¬ 
mitting filter is being used in the present units. This 
point merits further study. Volume compression to 
increase the modulation and a speech-scrambler to 
increase the security could be installed with little 
increase in weight. It should be emphasized that 
lamps of much higher power can be used with this 
kind of system to give much greater range with no 
increase in weight except for the power supply. A 
narrower frequency pass band might give an in¬ 
creased range with the present units. 

The type W system, without any major changes, 
would appear to be a very valuable and very secure 
supplement to portable radios for short-range mili¬ 
tary uses of all kinds. It has the best transmitter 
that has so far been designed for use in the HR 
(Section 4.8). 

44 ELECTRICALLY MODULATED ARC 
SYSTEMS 

4,4,1 Systems Not Developed under NDRC 

Systems for electrical modulation of a radiation 
source have previously not been able to compete 
very successfully with mechanical modulation of the 
emergent beam in such fields, for example, as sound 
motion pictures. This has been due, no doubt, to the 
lack of suitable gaseous discharge sources which can 
be efficiently modulated electrically. The carbon arcs 
used in the early Thirring system are noisy, have a 
low modulation ratio, and do not make a very 
steady or convenient source for field use. 6 However, 
in the last few years, electrically modulated radia¬ 
tion sources suitable for infrared communication 
have been produced in this country, France, and 
Germany, and have been used for military purposes. 
A French system using a xenon source will be de¬ 
scribed in Section 4.5.3. The German source is an 
r-f modulated mercury vapor lamp for infrared, 
but no further details are available. 

e See work of Contract OEMsr-1073, reported in the 
Summary Technical Report of Division 16, Volume 4. 




118 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


Two American electrically modulated infrared 
sources are now available: the “concentrated arc,” 
and the cesium vapor arc lamp (Chapter 1). 

Western Union Aural Signal Unit 

The 2-watt concentrated arc is used in the battery- 
operated aural signal unit developed by the Western 
Union Telegraph Company for the Army Signal 
Corps. The unit has a cesium phototube receiver, 
beam widths about 1 degree or less, and a voice range 
of the order of 2 miles. It will not be described here 
since it was not an NDRC development. 

Most of the further work on incorporating these 
two American electrically modulated sources into 
complete voice communication systems has been 
carried out by Northwestern University under Con¬ 
tract OEMsr-990. Complete systems have been built 
using the first source, and later the second, in devel¬ 
oping the so-called type E system for ship-to-ship 
communication (Section 4.4.2). Subsequently, this 
contract devised a third and a fourth system, em¬ 
ploying the cesium vapor lamp, for aircraft com¬ 
munication (Sections 4.4.3 and 4.4.4). These four 
systems will now be described. 

442 Ship-to-Ship Communication System 
Type E 

Description and Performance 

Course of Development. The type E system was 
developed under Contract OEMsr-990 (Project 
Control NS-159). The equipment as originally re¬ 
quested was to have angles of 15 to 40 degrees for 
secret communication at night in a convoy between 
one ship and several others simultaneously. When 
the development was initiated in May 1943, the 
intention was to maintain alignment of the trans¬ 
ceivers during communication by an automatic 
tracking arrangement; but this was found to be 
unnecessary since the beam angles of about 15 de¬ 
grees in the system as developed were sufficient for 
manual guiding of the heads, even on a destroyer 
in a fairly heavy sea. Voice and code ACW ranges 
of 5 miles were desired; these values were exceeded 
with the system developed. 

The type E system, as finally put into production, 
is somewhat more elaborate than the laboratory 
model shown in Figure 11. It consists of (1) two 
cesium-lamp sources and reflectors, (2) two TF-cell 
detectors and reflectors, (3) port and starboard trans¬ 


ceiver heads (Figures 12 and 15), each containing a 
source, receiver, reflectors, starting transformers, re¬ 
ceiver preamplifier, and an electron telescope for 
sighting, (4) two deck pedestals and gimbals (Fig¬ 
ure 15) for manual guiding of the heads (total 
weight of each pedestal, assembled, nearly 200 
pounds), (5) a starting circuit for the source, (6) a 
microphone and power amplifier for voice-modulat¬ 
ing the source, (7) a key and 1,500-cycle oscillator 
for sending code, (8) receiver amplifiers, head¬ 
phones, and loud-speaker, and (9) a control panel 
installed below decks (500 to 600 pounds), con¬ 
taining items 5, 6, 7, and 8 (Figure 16). 

The development may be divided into three stages. 
In the first stage, a 100- to 150-watt concentrated 
arc was used as the source and a gas-filled Cs-Ag-0 
surface phototube (Chapter 3) as the photodetector. 
In the second stage a great improvement in range 
was obtained by replacing the phototube with a 
Cashman TF cell developed by Northwestern Uni¬ 
versity Contract OEMsr-235 (Chapter 3) and by 
replacing the concentrated arc by the cesium-vapor 
lamp. The resulting system will be called here the 
laboratory model type E system. In the third stage, 
workers under Contract OEMsr-990 carried on con¬ 
sultation service for two manufacturers, Belmont 
Radio Corporation and Cover-Dual Signal Systems, 
Inc., Chicago, Illinois, engaged in adapting this 
system to quantity commercial production for the 
Navy. The latter stage was not part of the NDRC 
development and will not be described except in 
summary. Details of the consultation will be found 
elsewhere. 19 ’ 20 * 21 

Several reports have been written covering the 
laboratory development of type E. 13 ' 18 The last one 
contains in addition extensive data on the electrical 
and radiation characteristics and operating lives of 
the sources, comparisons of different types of photo¬ 
detectors and the theory of detector cell noise, 
studies of filters, and a discussion of ranges obtain¬ 
able with different values of beam width and source 
power. 

Laboratory Model. The successful laboratory 
model of the type E system is shown schematically 
in Figure 11. The system comprises a transceiver 
head on gimbals, a receiver-amplifier and power 
supply, and a main control panel. 

The transceiver head is shown in Figure 12 with 
the transmitter directly below the receiver. The 
filter has been removed to show the source, which 






ELECTRICALLY MODULATED ARC SYSTEMS 


119 



Figure 11. Type E communication system (schematic) 









































120 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


is a 90-watt cesium vapor lamp mounted axially at 
the focus of a 14-inch diameter, 1%-inch focal 
length, Alzak aluminum parabolic reflector. Most of 
the communication is carried by the two cesium 



Figure 12. Type E transceiver head, laboratory 
model. 


resonance lines 0.8521 and 0.8944 p. The hpi width 
of the emergent beam is about 13 degrees (Figure 
13) and the optics factor is about 40X- The trans¬ 
formers for heating the lamp filaments are mounted 
behind the mirror. 



Figure 13. Type E cesium lamp transmitter inten¬ 
sity distribution. 


The photodetector cell is a type B TF cell (Sec¬ 
tion 3.3.2) mounted axially at the focus of an iden¬ 
tical reflector. The hpr width is from 18 to 19 de¬ 
grees (Figure 14, curve A) and the optics factor is 
about 40 X- The cell is set in a socket in the cathode- 
follower preamplifier which is directly behind the 
mirror. 


The laboratory model transceiver head has dimen¬ 
sions of about 16x16x32 inches and weighs about 
60 pounds when mounted on gimbals for manual 
operation. The transmitter and receiver are sepa¬ 
rately shielded with sheet aluminum. The head is 
provided with a clamp for attaching a type C 3 
infrared electron telescope for sighting purposes. 

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DEGREES FROM AXIS 

Figure 14. Directivity patterns for various cell and 

mirror combinations. 

In this model short cables lead from the trans¬ 
ceiver head to the receiver-amplifier (Figure 11). 
This amplifier has voice and code pass bands which 
peak at about 1,500 cycles per second; the voice 
bandwidth is about 600 cycles per second, and the 
code band can be adjusted to a width of between 
10 and 200 cycles per second, just great enough to 
prevent the circuit from breaking into self-oscilla¬ 
tion. The voltage gain of the amplifier is about 
95 db on voice and 120 db on code. The receiver 
power supply and amplifier together weigh about 
15 pounds. Earphones are provided so that the man 
guiding the transceiver head and operating the 
receiver may monitor the conversation. 

Longer cables connect the transceiver head and 
receiver-amplifier to the control panel, which con- 



































ELECTRICALLY MODULATED ARC SYSTEMS 


121 


tains circuits for starting, d-c operation, and voice 
or code modulation of the cesium lamp. The panel 
will control and fully modulate either concentrated 
arc or cesium vapor lamps in the d-c power range 
between 75 and 150 watts. Modulators for lamp 



Figure 15. Belmont Radio Corporation US/E-2 
Nancy pedestal and transceiver head. 

sizes from 2 to 1,500 watts were also constructed 
during the development of type E. In order to 
increase the average speech modulation, the pre¬ 
amplifier has a volume compressor and a high-pass 
wave filter which cuts out frequencies below 1,000 
cycles. A send-receive switch is included which re¬ 
duces the cesium lamp current to about 1 ampere in 
the receive position and at the same time introduces 
a wave filter in order to reduce the interference due 
to radiation from the source back-scattered by the 
atmosphere into the receiver. 


The control panel is in a drip-proof case of dimen¬ 
sions about 12x18x24 inches and weighs about 130 
pounds. 

Noise-canceling dynamic microphones and crys¬ 
tal-type headsets are used. 

The power consumed by the whole system is 
about 550 watts from a ship’s 110-volt d-c supply, of 
which about 450 watts is dissipated in lamp ballast, 
and about 275 from the 110-volt 60-cycle supply. 

Production Models. The two models placed in 
quantity production by the Navy are very similar 
in their overall design to the laboratory model. Each 
has two transceiver heads (port and starboard), 
with provision for modulating the source in one of 
them at a time and for receiving from one or both. 



Figure 16. Prototype US/E-1 control panel with 
Cover-Dual signal systems incorporated. 

A Belmont Corporation pedestal and head is shown 
in Figure 15. The remainder of the electronic equip¬ 
ment is all in the control panel which is installed 
below decks. A control panel made by Cover-Dual 
is shown in Figure 16. 

The approximate weights of these systems were 




















122 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


given above under “Course of Development.” The 
total power consumption of each system is about 
700 watts (about 600 dissipated in lamp ballast) 
from the 110-volt d-c ship supply plus about 400 
watts from the 110-volt 60-cycle supply. 

Security. The XRN5PX PVA sandwich-type 
Polaroid filters and the OSU resin plastic filters 
made on white glass may be equally satisfactory. 
Extensive comparative measurements have been 
made on various filters to be used with the type E 
system, but no complete agreement about the abso¬ 
lute scale of the range measurements has been 
obtained. However, it appears that in order to re¬ 
strict the NVR to 400 yards, as desired for type E 
by the Navy, these filters must have ehT values for 
the cesium source of between 0.6 and 0.8. The maxi¬ 
mum range of detection of the transmitter by vari¬ 
ous foreign and American, type C 3 and C 4 , infrared 
electron telescopes has been tested and is of the 
order of the code range of the system. 22 This large 
range is of course necessary for manual guiding of 
the transceiver heads. 

Range. The range of type E is rather accurately 
known from numerous field tests. The vacuum voice 
range of both the laboratory model and the produc¬ 
tion prototype models is about 30 sea miles, ACW 
range 6.5 sea miles. Code ranges are more uncer¬ 
tain, as they depend on the sharpness and tuning of 
the receiver band, but the indicated vacuum range 
is about 100 miles, ACW range about 9 sea miles. 

The voice-range directivity pattern in various 
kinds of weather is shown in Figure 17. The area 
enclosed by the curvesds the area within which one 
ship can carry on simultaneous two-way communi¬ 
cation with several other ships. 

Evaluation. The type E system is comparatively 
easy to detect with an electron telescope, and intel¬ 
ligence transmitted by it may be received with sim¬ 
ple audio amplitude-modulation photoreceivers. This 
insecurity is a necessary consequence of the wide 
beam, the wavelength region used, and the simplicity 
of the modulation method and cannot be eliminated 
without going to an entirely different kind of system. 

Considerable thought has been given to maximiz¬ 
ing the performance of every component and the 
range of the type E system as a whole. Theoretical 
computations indicate that probably no other near 
infrared system, constructed with the same angles 
and power and present photodetector cells, can have 
more than about a mile additional ACW range. 


Gunfire causes little harm to sentence intelligibil¬ 
ity, but searchlights and starshells are troublesome. 
How much their effect can be reduced by filtering 
the receiver is unknown. Moonlight causes little 
trouble. The system was not designed for daylight 


IOO* 90* 80° 



Figure 17. Area of two-way voice communication 
with fixed transceiver, type E. 


communication but it might work in the daytime at 
medium ranges with filtered and shaded receivers 
and with minor changes in the preamplifier circuit 
or with the replacement of TF cells by PbS cells of 
the same size (see “Daylight Operation,” Section 
4.1.3). 

Transmitter 

Concentrated Arc Source. The concentrated arc 
source was used throughout the first stage of the 
development. Various sizes from 2 watts to 600 
watts were studied, with the emphasis concentrated 






















ELECTRICALLY MODULATED ARC SYSTEMS 


123 


on those in the 80- to 150-watt range. These studies 
were carried out by Northwestern University Con¬ 
tract OEMsr-990 in cooperation with Western 
Union Telegraph Company Contract OEMsr-984 
which produced the lamps. Many different design 
changes were suggested by workers in both groups 
and were made by those working under Contract 
OEMsr-984 in an attempt to increase the stability, 
life, and output of modulated filtered radiation. 

A typical 100-watt lamp has a voltage drop of 
about 16 volts at a current of 6 amperes, and an 
intensity of about 100 holocandlepower (hep) meas¬ 
ured on an RCA 919 phototube (Chapter 1). Since 
this is a cosine law source, the total flux is of the 
order of 300 hololumens. The lamp has a continuous 
spectrum peaking near 1 p, similar to that of a 
high-temperature tungsten lamp. 

The d-c or a-c variation of the radiant flux is very 
nearly linear with current, but not with voltage. 
The light-current modulation ratio depends some¬ 
what on the filter used and a great deal on the fre¬ 
quency (Chapter 1) but is about 0.10 at 1,000 
cycles per second with a Polaroid XR3X-41 filter. 
The ratio decreases at higher frequencies. Up to 
about 75 per cent current modulation can be used 
without serious distortion of the light wave. The 
arcs tend to be extinguished if the current drops to 
zero for a short time, as, for instance, by modula¬ 
tion near 100 per cent at frequencies below 1,000 
cycles per second. 

The apparent a-c impedance of the normal arc 
must be known in designing the transmitter ampli¬ 
fier; it is between 1.5 and 2 ohms at 1,000 cycles per 
second for the 100-watt arcs tested. The reactance 
component is about 0.5 ohm. 

Cesium Vapor Lamp. In the final laboratory 
model of type E, the cesium vapor lamp was used. 
Models of this lamp, produced by Westinghouse 
Electric Company under a BuShips contract, first 
became available to Contract OEMsr-990 in August 
1944. The 100-watt sizes proved to be similar in 
electrical characteristics to the 100-watt concen¬ 
trated arcs and could be operated from the trans¬ 
mitters designed for the latter with only minor 
changes in these transmitters. 

Some recommendations were made by those 
working under Contract OEMsr-990 as to the final 
design of a cesium lamp for the production type E 
equipment, based on the experience with early mod¬ 
els. The resulting lamp is the 90-watt 5.5-ampere 


Westinghouse CL-2 cesium vapor lamp which has 
been described in Chapter 1. 

A typical lamp has a voltage drop between 12 and 
20 volts at the rated current, depending on its gas 
pressure and previous history. The intensity is about 
100 to 150 hep measured on an RCA 919 phototube. 
This is almost a line source so that the distribution 
follows the sine law, and the total flux is 1,000 to 
1,500 hololumens. The lamp concentrates about 20 
per cent of its input energy into the two cesium 
resonance lines, 0.8521 and 0.8944 p. Very little of 
the radiation is in the “visible,” so the infrared 
filters used need not be very dense. 

The d-c or a-c variation of the radiant flux is 
again nearly linear with current. The light-current 
modulation ratio is 0.90 to 1.00, independent of the 
infrared filter used and of the frequency, up to 5,000 
cycles per second. Over 90 per cent current modula¬ 
tion may be used without serious distortion. The 
lamp does not extinguish with 100 per cent modula¬ 
tion, even at 60 cycles per second. 

The a-c impedance at 1,500 cycles per second is 
between 1 and 2 ohms. The reactance is about 0.5 
ohm. 

The cesium lamps differ from the concentrated arc 
lamps in requiring a warm-up period of about 15 
minutes after starting before they attain full inten¬ 
sity. 

These lamps have about three times as much 
hololuminous flux, about nine times as great a 
modulation ratio, and about twice as large an ehT 
(for filters giving a 400-yard visual range with the 
transmitters to be described) as a concentrated arc 
lamp of comparable wattage. The useful modulated 
and filtered infrared radiant flux from the cesium 
lamps is thus greater by a factor of about 50. Opti¬ 
cal systems to produce beams about 20 degrees wide 
are harder to design with high efficiency for these 
lamps than for the concentrated arc, so that the 
useful gain in practice is by a factor of about 20 
for phototubes and about 10 for TF cells, the longer 
wavelength response of which favors their use in the 
concentrated arc lamp. 

Lamp Operation. Both sources may be operated 
from the 110-volt d-c ship supply, with suitable 
ballasting in series to limit the current to the proper 
value. Part of the ballast resistance is made vari¬ 
able for manual adjustment of the current to the 
proper value. Iron-wire ballast tubes could be used 
for automatic current regulation, except that none 




124 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


of adequate power rating is commercially avail¬ 
able. 

Operation from a differentially compounded mo¬ 
tor generator, of the type used for welding arcs, has 
also been considered. This would be more efficient, 
eliminating the loss of power in ballast resistors, but 
would complicate the equipment. 

Whether the lamps are run from the ship supply 
or from a local generator they are modulated by 
the generator ripple. This may not interfere with 
speech modulation but may prove objectionable 
when a communication system is in the “receive” 
condition because of the noise in the receiver from 
the ripple in the backscattered radiation. If this is 
the case, the ripple may be eliminated by inserting 
L-C filtering in the d-c line. 

Lamp Starting. Both sources are started by 
high-voltage breakdown of the discharge path. The 
discharge then maintains itself when they are 
switched to the low-voltage, high-current power 
source. Several methods of applying the high voltage 
were studied, including the use of a Tesla coil or 
vibrator-induction coil, which is fairly satisfactory 
for laboratory use. 

The best starting method for the concentrated arc 
lamp in field operation is the use of a high-voltage 
rectifier with poor regulation. This provides a true 
arc of the desired polarity so that the switch-over 
to the low-voltage, high-current power source is very 
reliable. Even with this method several starts are 
often necessary before the arc strikes to the desired 
zirconium oxide cathode spot. An arc to any other 
spot is unstable and does not have the proper spec¬ 
tral distribution and modulation characteristics. 


R-l R-2 R-3 



ARC LAMP SOURCE 

Figure 18. Typical high-voltage rectifier starting cir¬ 
cuit for concentrated arc lamp. 

A typical rectifier starting circuit is shown in 
Figure 18. The rectifier voltage builds up to about 
4,000 volts d-c to insure breaking down the arc gap 
with every different type of concentrated arc. Either 
half-wave or full-wave rectification is satisfactory. 


In the final concentrated arc system, resistors R-l 
and R-2 limit the starting current to about 150 ma. 
Since R-2 is large (about 2,500 ohms), it absorbs 
little power when the lamp is modulated. 

During the development of type E, this same 
starting method was also used (associated with 
initial filament heating) for the cesium lamp, al¬ 
though it is said to shorten lamp life. A different 
method, recommended by the lamp manufacturer, 
was used for the cesium lamp in the commercial 
type E models. Still another method was employed 
in the aircraft systems (Sections 4.4.3 and 4.4.4). 

Lamp Modulation. Both sources are modulated 
by a constant-current generator, as shown in Fig¬ 
ure 19. The impedances of the d-c and starting 
circuits are kept high compared to the impedance 
presented by the lamp, so that they will absorb little 
of the modulator power. 


CONCENTRATED- 
ARC LAMP 


H) 


PREAMPLIFIER 


POWER 

PRC-0 


AMPLIFIER 

PRC - i 


PAC-1 



STARTING 

CIRCUIT 


POWER 

CONTROL 


Figure 19. Block diagram of complete type E trans¬ 
mitter circuits. 


For the 100-watt lamps of both kinds, pentode or 
zero-bias class B triode power amplifiers were used 
as modulators to provide an essentially constant- 
current source with high internal impedance and 
consequently good stability and linearity. 

Efficient power transfer was secured by using a 
transformer to match load impedances; the modula¬ 
tion energy was delivered through an electrolytic 
blocking condenser. 

The a-c volt-amperes required to modulate a lamp 
fully is approximately 


where I is the d-c current and Z is the lamp im¬ 
pedance. A 5-ampere lamp with 2-ohm impedance 
thus requires about 25 volt-amperes from the power 
amplifier. When transmission, transformer, and 
shunt losses are considered, the required modulator 
power may be 50 watts for such a lamp. 

































ELECTRICALLY MODULATED ARC SYSTEMS 


125 


The required modulator power for a cesium lamp 
may vary by 20 per cent, depending on the filament 
terminals to which the modulator is connected. 

Amplification. Amplification of speech waves 
from microphone to the final modulator stage is 
obtained in a conventional manner. A high-pass 
filter, with cutoff at 1,000 cycles, and a volume com¬ 
pressor are used to increase the average modula¬ 
tion current. 

A block diagram of the compressor circuit and a 
graph of typical performance are shown in Figure 
20. It is estimated that the use of the compressor 
raises the weaker speech passages about 6 db for 
the same modulation of stronger passages. This is a 
considerable gain as it is the stronger passages, of 
course, which limit the allowable modulation. 


A-C 


o o o 

CONTROLLED 
VARIABLE 
GAIN 
NPUT AMPLIFIER 
Q Q O 



t 


i 

F 

< 

cl 

ILTEI 

c 

) 

R 


t 



AG+DC 


CO 
RECTIFIER 

o 


—o 

A-C 


OUTPUT 


- i 

>— 

— O 


1 

AC 

O i 

> ( 

!> 

AMPLIFIER 

O 




A COMPRESSOR CIRCUIT B COMPRESSOR PERFORMANCE 

Figure 20. Block diagram of type E compressor cir¬ 
cuit, and graph of typical performance. 


To increase the modulation further, the higher 
speech frequencies are pre-emphasized by adjusting 
the modulator frequency response to increase at the 
rate of 4.5 db per octave. This compensates for the 
loss of amplitude in the higher human speech fre¬ 
quencies, so that all frequencies reach full modula¬ 
tion at about the same input signal level, and the 
total modulated radiant energy is brought to a 
maximum. 

The introduction of compensation for the decrease 
of modulation ratio with increasing frequency in the 
concentrated arc was considered, but it was con¬ 
cluded that this would decrease the total modulated 
radiant energy. 

Complete Transmitter Circuits. The block dia¬ 
gram in Figure 19 shows the functional arrangement 
of the transmitter units. 

The preamplifier provides the necessary amplifi¬ 
cation of microphone signals, so that the power am¬ 


plifier can fully modulate the lamp. The power 
amplifier contains all the wave filters, the volume 
compressor, and all frequency-response equalizing 
networks. 

The power amplifier provides modulating current 
for the lamp. A commercial 60-watt public address 
unit which was readily available and adaptable to 
rack mounting was used on all modulators. 

The starting circuit consists of a half-wave recti¬ 
fier and filter, as already discussed. 

The power-control unit contains all the arc bal¬ 
lasting resistors, switches, relays, and other required 
components for control of the system. All cables 
required to connect a complete control and modula¬ 
tor unit are brought to receptacles on this unit so 
that it serves to distribute all incoming and out¬ 
going power. 

Carbon microphones were used at first, but the 
batteries necessary to operate them were not suited 
for tests in freezing weather. Dynamic microphones 
were then tried but transmitted a great deal of dis¬ 
turbing ambient background noise in field opera¬ 
tions. Finally, noise-canceling dynamic microphones 
were used and proved very satisfactory. 

In the first preamplifier a clipper or peak limiter 
was put in to prevent extinguishing the concentrated 
arcs by overmodulation. This proved unnecessary 
in later circuits with better elimination of low fre¬ 
quencies. 

For code operation a two-tube, resistance-capaci¬ 
tance tuned, amplitude-stabilized oscillator was 
incorporated in the preamplifier. 

In the final volume compressor, a sharp turnover 
and relatively flat compression characteristic (Fig¬ 
ure 20) are obtained by use of a cathode follower 
to stabilize the rectifier bias so that the bias is 
independent of the level of the rectified and filtered 
signal. 

The final transmitter whose control panel is 
sketched in Figure 11 was capable of operating 
either a 100- to 150-watt concentrated arc or a 120- 
watt cesium lamp. Some improvements were made 
for use with the latter lamp. Filament preheating 
was provided. Duplex operation, which had pre¬ 
viously been used, was found to be no longer satis¬ 
factory because of backscatter. Therefore, a send- 
receive switch was installed in the control panel. 
This de-energized the receiver output in the send 
position and inserted ballast resistance and capaci¬ 
tance in the lamp line in the receive position, so as 

























126 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


to reduce the lamp current to 1 ampere and filter it. 
Further improvements in the final control panel 
included extensive shielding of the transmitter cir¬ 
cuits to prevent receiver-transmitter interaction. 

Receiver 

Phoiodetector Cells. Photodetector cells were 
chosen by several criteria. First, of course, high 
infrared sensitivity in microamperes per filtered 
hololumen was necessary. In order to obtain the 
large angles of view of 15 to 40 degrees desired 
for the equipment and still have a large enough 
mirror gathering area, large-area cells were needed 
(see Section 4.1.3). In the development of receivers, 
circuits were devised whose sensitivity was limited 
only by the shot noise or thermal dark noise in the 
detector cell itself, and therefore this cell noise had 
to be low compared to the sensitivity in order to 
give a large S/N ratio. 

Two different types of photodetector cell met 
these requirements fairly well and were used suc¬ 
cessfully in receivers during the development of 
type E. These are the gas-filled Cs-Ag-0 phototube 
(Chapter 3) which gave better results than vacuum 
phototubes. This tube was used in the first stage of 
of the development, the TF cell in the second. 

The most satisfactory gas-filled Cs-Ag-0 sur¬ 
face phototube which was commercially available 
for this work was the type CE-l-AB manufactured 
by the Continental Electric Company. It had a 
cylindrical cathode with a projected area of %xl% 
inches and a dark current of from 0.5 X 10 -11 to 
1.0 X 10‘ 10 ampere. The sensitivity of these tubes 
was of the order of 250 to 450 pa per hololumen for a 
source having a color temperature of 2848 K, and 
their response for this source with a Polaroid 
XR7X25 filter interposed was 10 to 30 per cent of 
that with no filter. 

Samples of several other types of gas-filled photo¬ 
tubes of various sizes were tested, including type 
CE-15 and CE-53 from Continental Electric Com¬ 
pany and type PIT from Farnsworth Television 
Corporation Contract OEMsr-1094, but all had 
lower average infrared sensitivity or higher dark 
current than the CE-l-AB tubes. Several Farns¬ 
worth type 6PEA six-stage multiplier phototubes 
(Chapter 3) were also tested. Such photomultipliers 
introduce the problem of a high-voltage power sup¬ 
ply but they simplify the auxiliary amplifier prob¬ 
lem. These tubes were found to give infrared S/N 


ratios up to the values obtained with the CE-l-AB 
tubes, but the cathode area was smaller, entailing 
a reduced angle of view, and for this reason they 
were not used further in the development. 

The Cashman TF cells w r ere not used until the 
spring of 1944 as it was felt that they would not be 
suitable for voice reception on account of their 
well-known falling frequency response (Chapter 3). 
When it was realized that the noise falls with fre¬ 
quency at the same rate, so that the S/N ratios are 
the same at the important speech frequencies near 
1,500 cycles per second as at low frequencies, TF 
cells w^ere immediately tried as voice detectors and 
proved very satisfactory. Type A cells, of %x%-inch 
sensitive area, were tried first, and gave S/N ratios 
with the concentrated arc covered with a Polaroid 
XR3X41 filter about as good as the best selected 
CE-l-AB gas-filled phototubes, which have some¬ 
what greater area. Upon request of Northwestern 
Contract OEMsr-990, Contract OEMsr-235 then 
produced larger TF cells, later called type B, about 
l 1 / 4x2 inches in sensitive area. Their sensitivities 
were almost equal to those of the smaller cells, and 
their larger size made it finally possible to obtain 
the large angles desired for the type E equipment, 
with mirrors of adequate size to give the desired 
range. Further discussion of these cells and their 
commercial manufacture is given in Chapter 3. 

The modulated flux required for threshold speech 
intelligibility is some 6 db less for the best selected 
phototubes than for the best selected TF cells of 
comparable area, using a cesium lamp source and 
the type E voice circuits; no conclusions about rela¬ 
tive performance under other conditions should be 
hastily drawn from this. 18 In spite of this poorer 
ultimate threshold, TF cells were chosen for type E 
because of several counterbalancing considerations: 
(1) The TF cells can be made more uniformly, so 
that their average performance seems to be better 
than the average phototube performance; (2) they 
can be made with larger areas than available photo¬ 
tubes of equal sensitivity, thus making feasible the 
large hpr angles desired for type E; (3) the TF cells 
are completely free from microphonics; (4) they are 
less sensitive to background light; and (5) their 
lower impedance level makes the problems of shield¬ 
ing and moisture less troublesome. 

For successful operation of the gas-filled photo¬ 
tube, which has a high internal resistance, moisture 
problems become so serious that completely enclosed 



ELECTRICALLY MODULATED ARC SYSTEMS 


127 


desiccated preamplifiers are necessary. With TF 
cells, which have a much lower dark resistance— 
only 1 to 10 megohms—treatment of tube bases and 
sockets with a water repellent is sufficient to prevent 
loss of sensitivity due to moisture. 

Preamplifier. A great number of preamplifier cir¬ 
cuits and modifications were constructed for a-c and 
d-c operation both with phototubes and with TF 
cells. The final circuits have a sensitivity limited 
only by the noise level of the cell itself, and thus 
represent a many-fold gain in sensitivity over the 
conventional circuits which were tried initially. 
The original reports 18 give the details of these low- 
noise circuits and the effect of various factors on 
their sensitivity. 

Early studies were made with d-c battery- 
operated phototube and TF-cell preamplifiers. The 
later and final laboratory preamplifier design for 
use with TF cells is a-c operated for convenience on 
shipboard. For reducing noise, the TF-cell load re¬ 
sistor is a wire-wound or equivalent low-noise com¬ 
position resistor, and care is taken to minimize 
microphonics. 

This preamplifier is essentially a cathode follower 
in which the cathode has been grounded to minimize 
any hum which might be introduced by heater 
cathode leakage or capacitance. The voltage supply 
is taken from the plate of the pentode and the cir¬ 
cuit is designed to prevent overvoltage on the 
photocell. 

Amplifier. The requirement of low noise in the 
amplifier is also stringent. The voice pass band must 
be restricted approximately to the region between 
1,000 and 2,000 cycles and the code pass band made 
as narrow as possible to cut out unnecessary noise. 
Care must be taken in selecting components, in pre¬ 
venting coupling between various components, and 
in eliminating common ground leads between high- 
and low-level stages. Helpful suggestions about the 
design were contributed by University of Michigan 
Contract NDCrc-185. 

Following early work with a d-c amplifier, 
the final four-stage a-c amplifier was constructed 
with the characteristics outlined above under “Labo¬ 
ratory Model.” A narrow pass band bridge-T feed¬ 
back amplifier for code has been incorporated in the 
main receiver-amplifier in such a way that it can 
also be used as the predominant factor in restricting 
the pass band in the voice position. A large fixed 
and a small variable resistor, in series with the 


choke in the bridge-T network, effectively lower its 
Q so that the bandwidth becomes 600 to 700 cycles. 
A switch shorts the fixed resistor for code reception, 
and the variable resistor is then adjusted to give the 
much narrower bandwidth desired for code. Trimmer 
condensers allow slight changes in the frequency for 
peak response. The frequency response of this ampli¬ 
fier for both voice and code is shown in Figure 21. 



500 700 IOOO 2000 3000 5000 


FREQUENCY IN C 

Figure 21. Frequency response on voice and code 

of type E a-c receiver-amplifier. 

Optical Systems 

Point Source Transmitter. Many systems were 
tried for obtaining a wide uniform beam from a con¬ 
centrated-arc source (first stage of development). 
In the best of these uniformity was obtained by use 
of spread lenses over a parallel beam formed by a 
parabolic mirror with the source at the focus. The 
most compact and efficient system of this type is 
represented in Figure 22 (T-3a). The source faces 
forward, that is, in the same direction as the emer¬ 
gent beam. It is placed at the focus of a “deep” 
parabola, the useful mirror surface of which is 
6 inches in diameter and 7 inches long, with a focal 
length of about 0.6 inch. An //1.2 plano-convex lens 
collects light that would otherwise pass uncollimated 
out the front opening of the mirror. An 18x8-degree 
spread lens was used to produce a divergent beam. 
The beam distribution is shown in Figure 23. The 
system had an optics factor of about 50X and effi¬ 
ciency of about 60 per cent. 

Vapor Lamp Transmitter. The system chosen for 
use with the cesium-vapor lamp (second stage of de¬ 
velopment) has been described above under “Labo¬ 
ratory Model.” It gave the best average intensity, 
throughout the central 20 degrees of beam width, of 
any of several different optical systems tried. The 
distribution from the transmitter is shown in Figure 
13. The pattern has radial symmetry, with a half- 
























128 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


T- 3 concentrated- 



arc, HOLOPHANE LENS R“1 6AS PHOTOTUBE 


(a) 18° X 8° 

O t = 50X 

.« (b) NO SPREAD LENS, 

ANGLE L2» 

O t = 3000 X 



X l4 -INCH SURFACE 


RECEIVING ANGLE 5°X8 e 
O r = 140 X 


CL-2 LAMP 


R-2 


TF CELL 



Figure 22. Optical systems, type E development. 


peak-intensity width of 13 degrees. The optics factor 
is about 40 X J it should be about three times this if 
the system were as efficient as the concentrated arc 

6 0 ------ 



Ol------ 

15 10 5 0 5 10 15 

DEGREES FROM AXIS 

Figure 23. Intensity distribution from concentrated 
arc in projector arrangement T-3a of Figure 22. 

systems just described. Much light is scattered at 
high angles: about 50 per cent of the total flux from 
the source falls inside a 50-degree circle, about 33 


per cent inside a 30-degree circle, and about 22 per 
cent inside a 20-degree circle. Attempts have been 
made to devise more efficient systems for use with 
this arc, but they all lead to greater weight, size, and 
complexity. 

The light in the central 20 degrees of the beam 
comes from only about 1^2 or 2 inches of the 3-inch 
arc column length. A lamp with a shorter column, 
such as the 50-watt cesium lamp (2-inch column) 
produced for the aircraft systems (see “Source,” 
Section 4.4.3) might be operated at lower power in 
this transmitter mirror and give almost the same 
maximum beam candlepower as the CL-2 lamp. 

Phototube Receiver. In the first stage of develop¬ 
ment, the receiver consisted of a 14-inch diameter, 
6-inch focal length, gold-plated, spun-metal, para¬ 
bolic mirror with the cesium phototube at the focus 
(Figure 22, R-l). With this system, the CE-l-AB 
tubes mentioned above gave an hpr field of view of 
5x8 degrees, the CE-15 tubes 4.5x29 degrees, the 
2x4-inch tubes about 10x20 degrees, and the 6PEA 
multipliers a circular field 5 degrees across. The 
angles may vary by 20 per cent from one tube to 
















































ELECTRICALLY MODULATED ARC SYSTEMS 


129 


the next, as a result of variations in the sensitivity 
pattern, irregularities in the glass ends, slight differ¬ 
ences in mounting, etc. 

Only the large tubes, which had poor infrared 
sensitivity, gave fields approximating the 15- to 
40-degree (presumably circular) width desired. The 
CE-l-AB system gave the best infrared response 
and was used in the tests in the first stage of devel¬ 
opment. A directivity pattern is shown in Figure 14, 
curve D. 

TF Cell Receiver. Since a TF cell is sensitive to 
light impinging on both sides, it may be mounted 
for optical efficiency parallel to the axis at the focus 
of a fairly deep parabola so that both sides are 
illuminated, as shown in Figure 22, R-2 and already 
described. Typical directivity patterns for various 
cells and mirrors are shown in Figure 14; hpr angles 
again may vary by 20 per cent from one cell to the 
next. The best response throughout the central 20 
degrees is given by the large l 1 /ix2%-inch type B 
TF cells mounted axially, in the same 14-inch mirror 
used for the cesium lamp transmitter. 

Filters. The least dense filters used in tests in the 
concentrated arc stage of development were Polar¬ 
oid XR3X41. They probably were more dense than 
necessary to give the 400-yard NVR desired, with 
the 18x8-degree, 5,000-hcp beam. The percentage 
response to the concentrated arc modulated at 1,500 
cycles through this filter was 6 per cent for a 
CE-l-AB phototube and 16 per cent for a TF cell, 
compared with the response to the unfiltered light 
from the arc. These percentages are lower than the 
ehT values because they include the effect of de¬ 
creasing modulation ratio for the arc at the longer 
wavelengths. It is estimated that these percentages 
might be three times as great for light transmitted 
by a filter chosen to give the maximum permitted 
visual range. 

The filters used in the final laboratory model of 
type E, using the cesium lamp with a 4,000-hcp 
transmitter beam, were of the Polaroid XR3X with 
ehT values from 30 to 60 per cent. Measurements 
made by Contract OEMsr-990 indicate that Polar¬ 
oid PVA filters and also recent OSU filters on white 
glass, with ehT values for the cesium lamp near 
50 per cent, give NVR values for type E near the 
permitted maximum of 400 yards. Measurements 
made at the Naval Research Laboratory [NRL] 
indicate that the acceptable ehT values may be 
allowed to go as high as 80 per cent. 


Assembled Equipment 

Two stages of development have been distin¬ 
guished. A typical assembly at the first stage of 
development, during the spring of 1944, included 
the following components: concentrated-arc source 
with large mirror-spread lens system, transmitter in 
open racks, carbon microphones and peak clipper, 
duplex operation, phototube detectors at focus of 
shallow mirror, tubes exposed to moisture, d-c re¬ 
ceiver operation from batteries, dense filters, and 
transmitter and receiver heads on separate gimbals. 

The summer of 1944 brought a transitional stage. 
A typical assembly then included more compact 
concentrated-arc transmitter optics, transmitters in 
drip-proof cases, dynamic microphones, better high- 
pass filters, phototubes in desiccated cases, TF cell 
receivers, a-c receiver operation, heads on a single 
set of gimbals, infrared telescopes for guiding. 

By the fall of 1944 the cesium lamps had become 
available and the second stage of development, the 
final laboratory model of type E, was completed. 
The components of the assembly are given earlier 
in this section. 

Interrelations of Components. Table 3 lists what 
are believed to be representative values of the total 
performance level of the two kinds of detectors with 
the two different sources and with various filters. 
The values may vary by several db from one lamp 
or one cell of the same kind to another. The signal 
level is given in decibels above the voice communi¬ 
cation threshold (which is different for each cell- 
source combination). The measurements were taken 
with the bare cells in the uniform beam at an effec¬ 
tive distance of 105 yards from the bare sources, 
which were fully modulated at 1,500 cycles per 
second. The circuits were the type E voice circuits. 
The measurements on the phototube have been 
increased 4 db [corresponding to the square root of 
the ratios of the areas of the cells, see Chapter 3] 
to compensate for its small size. From the observed 
signal level and distance, if the optics factors are 
known, the maximum bare-source, bare-cell range 
for threshold voice communication can be computed, 
and also the range for a complete system using such 
sources and cells. 

In the first column of Table 3, the difference 
between the first and third (or second and fourth) 
lines gives essentially the modulation ratio of the 
concentrated arc, as the two lamps have about the 



130 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


same holocandlepower and the cesium lamp has a 
modulation ratio of unity. For each line of the 
table, the difference between the first column and 
column (X) gives approximately the usable filter 
holotransmission for the different sources when 
mounted in the chosen transmitter systems. 

Table 3. Total response of several combinations of 
sources, filters, and detectors (100-watt source fully 
modulated at 1,500 cycles). Figures show decibels 
above threshold voice signal; bare sources and bare 
cells at 105 yards. 




1 

2 

3 

4 

5 



No Wratten 


XR3X- 

XR7X- 



filter 

87 


41 

25 


Source Detector 

(db) 

(db) 

(X)* 

(db) 

(db) 

1 . 

Concen¬ 
trated arc TF cellt 

29 

24 

(21) 

13 

12 

2. 

Concen¬ 
trated arc Phototube* 

35 

26 

(23) 

10 

4 

3. 

Cesium 

lamp TF cellt 

47 

45 

(44) 

38 

34 

4. 

Cesium 

lamp Phototube* 

54 

52 

(51) 

44 

38 


* Estimated signal level with filters giving NVR of just 400 yards with 
transmitter systems T-3a and T-4 (Figure 22). 

t l 1 /4x2%-inch grid, S/N = 57 db on U of M test set (Chapter 3). 

♦ CE-l-AB (gas filled), %xl 1 / 4-inch surface; values corrected to 
larger size. 

The values in column 3 are the most important 
for they indicate the comparative excellence of dif¬ 
ferent source-cell combinations with optimum filters 
for sources or transmitters having the same holo¬ 
candlepower. The cesium lamp-phototube combina¬ 
tion is seen to be the best, but the cesium lamp-TF 
cell combination was chosen for the reasons indi¬ 
cated above under “Photodetectors.” The total per¬ 
formance level of this combination is seen to be 
better by a factor of about 10 than the concentrated 
arc combinations, and thus corresponds to a factor 
of about three in vacuum voice range. 

Operational Tests 

Concentrated Arc System. Thirty-four night field 
operations were recorded during the development 
of type E. The first 22 of these were tests of the 
concentrated arc system, carried out in the period 
August 1943 to August 1944, the first 16 being over 
land and the remaining 6 over Lake Michigan. In 
all of these concentrated arc tests, one transmitter 
or transceiver was placed on the roof of North¬ 
western University Technological Institute, which is 
in Evanston on the shore of the lake, and the re¬ 


ceiver or transceiver was located, for the tests over 
land, either in Grosse Pointe Lighthouse, 0.5 mile 
away, or at various beaches in Chicago and Evans¬ 
ton, up to 6 sea miles away. In the tests over water, 
transceivers were placed on the Navy lighter 
YF-538 which was made available for these opera¬ 
tions by the Ninth Naval District. The customary 
procedure on the lake tests was for the ship to set 
out from the Naval Armory in Chicago, 12 miles 
away from the Institute, and sail toward the Insti¬ 
tute until good voice contact was established with 
the shore unit. The course of the ship was then 
reversed and as it returned to the Armory, voice and 
code maximum ranges were determined on each leg 
of the trip. 

The first reasonably successful communication 
was obtained over land in September 1943 between a 
100-watt concentrated arc, mounted in an 18-inch 
parabolic reflector without filter, and a vacuum 
phototube receiver in a 14-inch reflector, connected 
to an amplifier having 70 db gain. The hpi angle 
was about 3 degrees and the range limit was found 
to be 4.1 miles for voice communication. Later, 
using the same source and a gas-filled phototube 
in a 14-inch reflector giving hpr angles of 5x8 de¬ 
grees, good voice communication was obtained at 
4 miles, and contact was established as far away as 
6.8 miles, in a misty atmosphere. 

Ship-to-shore tests in December 1943 with similar 
equipment gave 4.3 sea miles voice range, with 
5-degree hpi angle and 5x8-degree hpr angles as 
above, with transmission about 0.6 per mile. This 
corresponds to a 20 sea mile vacuum voice range. 
With transmission 0.4 per mile, and 3.5-degree hpi 
angles, the ranges were 4.6 miles for voice (30 miles 
vacuum) and 8.9 miles for code, although the com¬ 
munication was carried on through ice on the pilot 
house windows. 

In a ship-to-shore test on April 18, 1944, with the 
favorable transmission of 0.7 per mile, with 100- 
watt concentrated arcs in spread-lens systems and 
with the above receiver, the voice range was about 
6.5 sea miles, and code was received until the line 
of sight was interrupted by the Navy pier at 10.6 
sea miles. In this test, both 6xl0-degree and 4x12- 
degree hpi angles were used, with XR7X filter. The 
5x8-degree receivers were again used. 

With the same equipment (except the filter, 
which was replaced by an XR3X type) in another 
test on May 15, under terrible weather conditions 










ELECTRICALLY MODULATED ARC SYSTEMS 


131 


with threatening rain and a transmission of about 
0.1 per mile, the range was 2.2 miles for voice and 
3.6 miles for code transmission. When the April 
and May ranges are reduced to vacuum conditions, 
however, the ranges for voice communication both 
came out at about 15 miles. This value is less than 
that of the December 1943 tests because of the use 
of filters and the widening of the beam in the later 
tests. During the May 15 test, high humidity pro¬ 
gressively reduced the performance of the photo¬ 
tube receivers because of increased surface leakage. 
This experience was a major factor in stimulating 
consideration and use of TF cells instead of photo¬ 
tubes. 

Two further concentrated arc ship-to-shore tests 
w r ere made in August, 1944, using the new 18-degree 
hpr width TF cell receivers (which were in essen¬ 
tially their final laboratory-model form) and the 
final 8xl9-degree hpi width concentrated arc spread- 
lens transmitters (Figure 22, T-3a). The ranges for 
voice were about 2.2 sea miles and the ranges for 
code 3.0 to 5.6 miles, with 0.5 to 0.8 transmission 
per mile. The range for voice communication in 
vacuum is thus about 8 sea miles, and the ACW 
range 3.5 sea miles. The filters used were XR3X 
on one test, XR7X on the other. The further loss in 
vacuum voice range is the result of the further 
increase made in the beam angles in an attempt 
to meet the original request for 15- to 40-degree 
angles. 

Cesium Vapor Lamp System. In the August 31 
test, the last of those just mentioned, one of the 
recently received cesium lamps was operated as a 
source for the first time. Fortunately, these lamps 
and the concentrated arcs were similar enough in 
electrical characteristics to be operable from the 
transmitter panels then being used for the latter. 
These same control panels with only slight modifi¬ 
cations were therefore used throughout the remain¬ 
der of the cesium lamp type E laboratory devel¬ 
opment. 

The cesium lamp was placed for this test in an 
improvised optical system with 40-degree hpi angle, 
a system which later proved to have been very in¬ 
efficient. Nevertheless, the lamp immediately showed 
its superiority over the concentrated arc, a result 
which had been expected from preliminary labora¬ 
tory observations (see above, “Cesium Vapor 
Lamp” and “Interrelations of Components”). This 
cesium lamp system gave voice and code ranges of 


3.8 and 5.0 sea miles, respectively, on this test, in 
0.5 transmission per mile weather, at the same time 
that the final model of the concentrated arc system 
was giving 2.2 and 3.0 mile ranges. The vacuum 
voice ranges are 12 sea miles for the 40-degree hpi 
width cesium lamp system compared to 8 sea miles 
for the 8xl8-degree concentrated arc system. Since 
a sufficient area of a suitable filter could not be 
obtained for this test of the cesium lamp, a thick 
cover glass was used as a substitute. 

Immediately following this test suitable cesium 
lamp optical systems were built up and the trans¬ 
mitters were modified to give better starting and 
more reliable operation of the vapor lamps. The 
system was thus put into essentially its final form, 
as already described under “Laboratory Model,” 
with 13-degree hpi and 18-degree hpr (TF cell) 
widths. No further changes were made in these final 
models except for some alterations in the filters 
used and some small changes in the preamplifier 
circuit constants. 

The remaining 12 field tests, using this final 
system, were carried out in October and November 
1944. The first three of these operations were ship- 
to-shore tests between the Institute roof and the 
YF-538 on the nights of October 4, 5, and 6. All 
the results of these tests were essentially the same, 
the vacuum voice range being about 17 sea miles, 
the ACW range 5 sea miles. Variations in trans¬ 
mission from 0.4 to 0.7 per mile on the three nights 
produced variations in actual voice range from 3.4 
to 5.3 sea miles and in code range from 4.0 to 7.0 
sea miles. The filters used were of XR3X41 Polaroid 
cellophane type with an ehT of 0.30 for the cesium 
lamp. 

During the October 6 test, a representative from 
BuShips and members of Section 16.4 were present. 
A direct and careful comparison was made between 
the performance of the cesium lamp system and 
that of the final 8xl8-degree concentrated-arc sys¬ 
tem. The latter gave 2.9 and 3.0 sea miles voice 
and code ranges (8 miles vacuum voice range 
again) at transmission 0.7 per mile, as compared 
with 5.3 and 7.0 sea miles for the cesium lamp 
system (17 miles vacuum voice range). 

On this occasion measurements were also made, 
at a fixed range of 2 miles, on the maximum angle 
through which a transmitter or a receiver at one 
station could be swung and still maintain intelli¬ 
gible communication while the transceiver at the 



132 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


other station remained fixed in direction. The con¬ 
centrated arc transmitters could be swung over a 
total angle of about 18 degrees, as expected from 
the width of the spread lens beam in the horizontal 
plane; a TF cell receiving from this arc system 
could be swung over about 29 degrees. The cesium 
lamp transmitter had a total beam width, measured 
in this way and at this range, of 48 degrees and the 
TF cell receiver in this case had an angling view of 
about 40 degrees because of the stronger signal. 
Variations in individual judgment produce varia¬ 
tions of several degrees in these widths, just as they 
produce variations of a mile or more in range deter¬ 
minations; however, with experience, such observa¬ 
tions become more consistent. These measurements 
of beam widths at 2 miles agree qualitatively with 
laboratory measurements (see Figures 13, 14, and 
17), but no quantitative comparison can be made 
as the laboratory work was usually not carried to 
angles as large as these, where the intensity (or 
response) has fallen some 30 db from the peak. 

Cape Henlopen Tests. Following this October 6 
demonstration of the superiority of the cesium lamp 
system, this final laboratory model type E equip¬ 
ment was taken to the BuShips test station at Cape 
Henlopen, where 9 further field tests were made in 
cooperation with BuShips on October 18 and 19 
and on November 14 to 28, 1945. 16>17 A vacuum 
voice range of 30 sea miles, ACW range 6.5 sea 
miles, represents all of the results with fair accu¬ 
racy. The improvement from the earlier October 
results may be due partly to a change of filters to 
Polaroid Cycle-Weld-bonded XR3X-55 nylon-base 
filters with ehT values of 0.50 and partly to small 
improvements in receiver circuits. Part of the dif¬ 
ference in the early and late October ranges may 
be fictitious, since the earlier values were low be¬ 
cause of overestimates of atmospheric transmission 
by the visual method used for the Lake Michigan 
tests. The Cape Henlopen vacuum ranges should 
be fairly reliable, as instrumental determina¬ 
tions of T were made during these tests by several 
different methods. 16 ’ 17 The actual voice and code 
ranges in the Cape Henlopen tests varied from 3.5 
and 4.6 sea miles on November 15, when the trans¬ 
mission was 0.30 per sea mile, to values limited only 
by the horizon at 7.5 miles on November 18, with 
transmission 0.65 per sea mile; code ranges were 
limited only by the horizon on every test except 
that of November 15. 


All the Cape Henlopen operations were ship-to- 
shore tests between the BuShips test station and 
the USS Marnell except that of November 18, 
which was a ship-to-ship test between the Marnell 
and the minesweeper YMS 462. The equipment was 
officially demonstrated on this occasion to represen¬ 
tatives of the following branches of the Armed 
Services: COMINCH, SONRD, BuShips, NRL, 
BuAero, SSL, Signal Corps, AAF, and the WDLO 
with NDRC, as well as the British Admiralty Dele¬ 
gation. On this demonstration, the effect of lights 
and gunfire on the equipment was observed also. 

The following paragraphs give a summary of the 
performance characteristics of the equipment as 
determined from the Cape Henlopen Tests. 

Code Transmission. In eight out of nine operations in 
November the range for code transmission was limited only 
by the horizon (about 14,000 yards). On November 15, the 
atmosphere was hazy and the transmission values varied 
from 0.25 to 0.30. The limiting code range was 4.6 sea miles, 
corresponding to a vacuum code range of R v = 75 sea miles. 
Only on this occasion was it possible to test code transmission 
speeds at distances beyond the limiting range of voice com¬ 
munication. The following quotation is taken from the 
report of this test issued by BuShips: 17 “Two messages were 
transmitted using code at over 20 words per minute, one of 
the messages being composed of arbitrary letter groups. The 
reception of both messages was nearly 100 per cent, with the 
errors due to the operators rather than to the lack of ability 
of the equipment to transmit at these speeds. In the opinion 
of the operators, at this signal level (7,100 yards, 0.25 trans¬ 
mission) the equipment can be keyed as rapidly as desired.” 
The signals at 1,500 cycles came in very distinctly without 
the usual static background experienced in radio coding. 

Ship-to-Ship Tests. The values of angular spread are suffi¬ 
cient for manual alignment to insure uninterrupted ship-to- 
ship voice communication during rapid maneuvering of 
ships, and when ships are experiencing heavy rolls. This was 
proved by the ship-to-ship tests carried out on November 18. 

One projector was mounted on the USS Marnell, the 
other on the minesweeper YMS 462. Transmission was 0.64 
per sea mile. The sea was rough and both ships rolled and 
pitched considerably. At 13.400 yards, when voice communi¬ 
cation was still 100 per cent, both ships started making S 
turn, the Marnell (ahead) making 10 knots, the YMS 
(behind) 5 knots. A hard right and then hard left rudder 
swung the ships some 70 degrees off the base course on either 
side. Communication was continuous until the beams were 
interrupted by the ships dipping behind the horizon (7.2 
miles) in the troughs of waves. 

Official Demonstration. On the night of November 16, a 
demonstration was given which was attended by 19 repre¬ 
sentatives from the Army, Navy, and NDRC. Ship-to- 
shore communication was carried out between the USS 
Marnell and the Bureau of Ships test station. Voice recep¬ 
tion was excellent up to 4.5 miles; at this range code trans- 



ELECTRICALLY MODULATED ARC SYSTEMS 


133 


mission was begun and carried out to the horizon limit at 
about 8.0 miles. 

At 1.7 miles and at 2.65 miles the Marnell was anchored 
in position. At each position, six 40-mm guns fired tracers 
from Fort Miles in the direction of the ship and two star 
shells were fired from the Marnell. Prior to each firing 
several 60-inch Army searchlight beams were swept across 
the bay between shore and ship. 

Searchlights. A whining and growling sound is produced 
in the receiver by light from searchlights. This probably is 
due to the sputtering of the carbon arc. As the beam is 
swung toward the receiver, the noise rapidly increases and 
blanks all other reception. The receiver is completely 
deadened when directly illuminated by the searchlight beam. 

Gunfire. On the ship gunfire and shell bursts merely 
produced clicks in the receiver. At the shore receiver each 
flash from the 40-mm and 90-mm guns momentarily 
“blocked” the receiver. The effect of searchlights and gunfire 
could probably be greatly reduced by the use of an infrared 
filter over the receiver similar to that over the transmitter, 
with almost no loss in signal intensity. Three out of four 
listeners reported sentence intelligibility as still “very good.” 

Star Shells. At the test station, the star shells were in the 
field of view of the receiver and produced a low frequency 
rumble which cross-modulated with voice signals from the 
ship and ruined intelligibility. 

Other Lights. An X-2A (infrared) beacon on shore caused 
a hum which interfered very little with reception at the 
shore station. Blinker communication lights on ships off 
shore did not interfere. 

Conclusions. The following is an evaluation of the per¬ 
formance of type E equipment by the Bureau of Ships per¬ 
sonnel: 17 

a. The range of the type E equipment is greater than 5 
nautical miles for average weather conditions (based on 0.6 
transmission for average conditions in the Pacific area). 

b. Since the rolling and pitching of the two ships on the 
night of 18 November is considered equivalent to that which 
might be experienced by a destroyer in a fairly heavy sea, 
the beam angles are sufficiently large to maintain communi¬ 
cation under mast weather conditions when the equipment is 
installed on larger vessels. 

Present Status 

Following the successful tests at Cape Henlopen, 
the Navy undertook a procurement program with 
Belmont Radio Corporation and with Cover-Dual 
Sign Systems, Inc., both of Chicago, for quantity 
production of the type E equipment. Those working 
under Contract OEMsr-990 were authorized by 
OSRD to furnish consultant and advisory service 
for the manufacturing program. 

At this writing, one prototype model from each 
manufacturer (Figures 15 and 16) has been tested 
by those working under Contract OEMsr-990 both 
in the laboratory and in a field operation. These 
models have been described in the summary above 


under “Production Models.’’ Their general perform¬ 
ance characteristics are essentially the same as 
those of the final laboratory models, except that 
they are built to Navy specifications and must pass 
the usual vibration, shock, temperature, corrosion, 
and weathering tests. Manufacture is continuing. 

The laboratory equipment and the completed 
laboratory systems assembled under Contract 
OEMsr-990 have been transferred from OSRD to 
the Navy for continuation of the work of this con¬ 
tract at Northwestern University under Navy Con¬ 
tract NObs-28373. 

Recommendations 

Much work has been done on maximizing the 
performance of all the components of the type E 
system up to the limits practical for field operation. 
It is believed that the night ranges of this system 
are within about 1 mile of the maximum ACW 
ranges theoretically obtainable in the NIR with 
100-watt sources of any kind, beam widths of about 
15 degrees, and any existing detector cells. 

It is not certain whether the bandwidths used in 
the transmitter and receiver circuits in either the 
laboratory or production models of type E really 
give optimum intelligibility and total performance 
for the system. It is thought that a few db may be 
gained by improvements at this point. Further im¬ 
provement might be made by redesign of the trans¬ 
mitter optical system for greater efficiency of light 
output in the central 20 degrees and less radiation 
at wide angles. This would increase the central 
ranges and decrease the widths of the patterns 
shown in Figure 16; whether this is desired by the 
Navy is not known. 

Some consideration might be given to adapting 
the type E system to daylight use (without loss of 
its night range) as mentioned under “Evaluation” 
above. 

The xenon source (Section 4.5.3) should be 
studied as an alternative to the cesium lamp source. 

A comparison should be made of the weight, 
power, range, and stability of mechanically modu¬ 
lated systems and electrically modulated systems 
for shipboard use. 

The possible amalgamation of type E with an 
identification system such as type D is discussed 
in Section 5.2. 

Probably no wide-beam, audio-amplitude-modu¬ 
lated, near-infrared system will long remain secure 



] 34 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


against detection and possible interception of mes¬ 
sages during military use in close contact with the 
enemy. A security device which can be incorporated 
in the present general design of type E would be a 
speech-scrambling system. Frequency modulation 
using a fairly low-frequency carrier wave (say 
about 20,000 cycles) has also been considered and 
would be possible, though perhaps inefficient, with 
the cesium vapor source; this would probably re¬ 
quire the replacement of the TF cells in the receiver 
by phototubes. 

Further improvement in security can be obtained 
only by going to narrow-angle systems, and/or to 
different wavelength regions such as the inter¬ 
mediate infrared (Section 4.8) or perhaps the ultra¬ 
violet. Use of narrow angles of course introduces 
the complications and limitations of associated 
tracking devices. 

Conversion of type E to a narrow-angle system 
was considered by those working under Contract 
OEMsr-990. Simple replacement of the lamp and 
cell by a 94-inch long cesium lamp and a %x%- 
inch TF cell, both of which have been produced 
experimentally (Chapters 1 and 3) would give a 
4- to 6-degree system with no other changes. Use of 
the final concentrated-arc transmitter optical sys¬ 
tem (6 inches in diameter) without spread lens 
(T-3b in Figure 22), and a ^x^-inch TF cell with 
a 6-inch diameter 4-inch focal length mirror, would 
give a compact system. With angles of 1 to 3 de¬ 
grees this could be operated directly from the pres¬ 
ent type E control panel; the range should be about 
the same as for present type E. 

443 Aircraft Systems; Plane-to-Ground 
Communication System (P-G) 

Course of Development of Aircraft Systems 

Following the successful completion of the 
cesium lamp type E system for ship-to-ship com¬ 
munication, Northwestern University Contract 
OEMsr-990 was assigned in March 1945 to the 
problem of adapting the same principles to the 
development of two similar voice communication 
systems for use in Army aircraft. 24 

The first system was initiated under Project 
Control AC-226.03, requested by the Army Air 
Forces. It was intended to provide voice communi¬ 
cation at night between a hand-held unit on the 


ground and a hand-guided unit pointing out of the 
window or hatch door of a plane flying at between 
500 and 1,000 feet altitude at about 125 miles per 
hour. The system was to be for use in situations 
involving the maintenance of communications and 
transport of supplies by air at night to guerrillas, 
paratroops, or other isolated ground troops. The 
ACW range was to be 3 miles. 

Tests conducted at Wright Field indicated that 
minimum transmitting and receiving angles should 
be about 8 degrees ±2 degrees and 10 degrees 
respectively, for the ground unit and about 15 
degrees ±5 degrees and 10 degrees, respectively, 
for the plane unit. These angles are necessary for 
making contact and maintaining continuous com¬ 
munication between hand-held units using auxiliary 
infrared electron telescopes for visual sighting. 

A requirement was that the complete ground unit 
weigh less than 40 pounds, so that it could be car¬ 
ried to earth on the person of a paratrooper and 
that it have a self-contained power supply. This 
made it impossible to have the plane and ground 
units nearly alike, as had been originally desired, 
and still achieve the specified range and angles. 
Mechanical modulation was indicated for the 
ground unit, as this requires only very lightweight 
equipment, although the efficiency is low. It was 
decided that, by way of compensation, the plane 
unit (P-G system) should have a larger receiver 
mirror and an electrically modulated source oper¬ 
ating from the aircraft power supply and that the 
weight of this unit (exclusive of cables) should be 
allowed to go as high as 60 pounds. 

The development of the ground unit was allo¬ 
cated by Section 16.4 NDRC to University of 
California Contract OEMsr-1073, as this unit was 
already engaged on a similar problem. The com¬ 
pleted ground unit, type W, has been described in 
Section 4.3.2. The data on range tests between the 
completed plane and ground units are given below. 

The second aircraft system was designed under 
Contract OEMsr-990 as Army Air Forces Project 
Control AC-226.04. It was to provide voice com¬ 
munication between the adjacent planes of a 
bomber (B-29) formation under conditions requir¬ 
ing radio and radar silence. In two important re¬ 
spects this plane-to-plane (P-P) problem was a 
greater departure from the type E development 
than was the plane-to-ground (P-G) problem. The 
system was to be an all-around one, communicating 





ELECTRICALLY MODULATED ARC SYSTEMS 


135 


over angles of approximately 120 degrees horizon¬ 
tally by 60 degrees vertically at the front and also 
at the rear of a bomber. Also, it was to operate in 
daylight with the receiver illuminated by the equiv¬ 
alent of an average overcast north sky, the range 
to be at least % mile day or night. The weight of 
the whole system (exclusive of cable) on one plane 
was to be less than 120 pounds. 

Electrically modulated sources were evidently 
required. It was decided, after inspection of a 
bomber of the B-29 type for which the equipment 
was to be designed, that the physical layout of the 
ship would require two transceiver units in the nose 
and one compact one in the tail in order to give 
the desired communication angles. 

In constructing the equipments for these two 
projects, components were selected to meet Air 
Force specifications as far as possible, since the 
Army wished to put them into manufacture with 
the least possible revision and delay if they worked 
satisfactorily. This procedure makes it possible to 
predict accurately the weight, size, and mechanical 
and electrical performance of possible manufac¬ 
tured units. It also minimizes any adverse effects 
of low pressure, moisture, or cold which might be 
encountered in actual flight tests on the labora¬ 
tory models. 

When the end of World War II brought termina¬ 
tion of the contract, neither of the two systems had 
been completed in the form intended. The P-P 
system had been considered more important by the 
Army at first, and an electronic power supply unit 
and control panel for operating three sources from 
aircraft voltages were finished by July 1945. Several 
questions connected with receiver design and the 
best use of photodetector cells for daylight com¬ 
munication had not been settled, and the trans¬ 
ceiver heads were therefore not finished when, as a 
result of the changing military situation, the Army 
began to emphasize the urgency of completion of 
the P-G system. The transceiver unit for this sys¬ 
tem was promptly finished. To save time it was 
operated immediately in laboratory range tests 
with the type W unit using the three-lamp control 
panel already constructed. This combination of 
units was then shipped to Wright Field for actual 
tests between a plane and the ground. The end of 
the war occurred before a separate control panel 
could be built for the plane-to-ground unit. 

In what follows estimates will be given as far as 


possible of the performance of the equipment if it 
is finished as designed, based on the performance of 
the completed units. 

Description and Performance 

General Design. The general design of the P-G 
system is not unlike that of type E (Figure 11) if 
the small separate amplifier box in the receiver line 
is replaced by a starting transformer box in the 
source line. 24 One complete aircraft installation 
should consist of (1) a cesium lamp source, reflec- 



Figure 24. P-G transceiver head. 


tor and filter, (2) a starting transformer box, (3) a 
photodetector, reflector, and preamplifier, (4) a con¬ 
trol panel containing transmitting and receiving 
circuits designed for connection to the plane’s inter¬ 
communication system, and (5) a power supply, 
which may be incorporated in the control panel. 

The transceiver head is shown in Figure 24. The 
infrared filter has been removed to show the source, 
which is an electrically modulated 50-watt cesium 
vapor lamp mounted axially at the focus of a l 1 /^- 
inch diameter, %-inch focal length, Alzak alumi¬ 
num parabolic reflector. The hpi width of the 





136 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


emergent beam is 16 degrees and the transmitter 
optics factor is 18X* 

The photodetector is a type A %x%-inch TF cell 
mounted axially at the focus of an identical re¬ 
flector which is covered by the electrostatic screen 
seen in Figure 24. The hpr width is about 12 degrees 
and the receiver optics factor 40X- The cathode- 
follower preamplifier is mounted in the back of 
the transceiver head. 

In an actual installation the head would presum¬ 
ably be placed in gimbals or supported so as to 
rotate freely; it would point out of the hatch door 
on the left side of a cargo plane. An infrared elec¬ 
tron telescope would be attached to permit an oper¬ 
ator to sight visually on the ground unit. 

A short cable connects the head to the starting 
transformer box and longer cables, if desired, to the 
control panel. The control panel was to be designed, 
as the three-lamp control panel of the P-P system 
is, to operate from the carbon microphones and to 
feed the headsets of the plane’s normal intercom¬ 
munication system so that the pilot or copilot could 
carry on the conversation. The conversation would 
presumably be monitored by the man guiding the 
transceiver head who might also operate the con¬ 
trol panel. 

The dimensions and weights of the units (exclu¬ 
sive of cable) would be: 


Unit 

Transceiver head 
Starting box 

Control panel 
Power supply 

Total weight 


Dimensions 
8x8x16 inches 
4x6x8 inches 
Combine in single unit 
less than 8 x 15 x 20 
inches (size of com¬ 
bined 3-lamp P-P 
unit) 


Weight 

101b 

101b 


201b (est.) 
17 lb (est.) 


57 lb 


The power consumption of the whole equipment 
would be about 120 watts from the 28-volt d-c plane 
supply plus an estimated 130 watts from the 115- 
volt 400-cycle a-c supply. 

Security. The filter used over the transmitter is 
the Polaroid XRN5PX-55 PVA sandwich type. 
The XVR is computed to be about 230 yards from 
the measurements of Contract OEMsr-990 and 
about half this if determined by NRL methods 
(see “Filters,” Section 4.4.2); it probably can be 
taken as equal within experimental error to the 
value of 400 feet specified in Project Control 
AC-226.03. The vacuum range for detection of the 
transmitter by a type C 4 infrared electron telescope 


is expected to be about 35,000 yards, and the ACW 
range about 10,000 yards. Since the telescopes are 
used for guiding it is desirable that this range be 
large. It is about 30 per cent larger than the com¬ 
munication range between the P-G unit and the 
type W ground unit so that the ground unit should 
have little difficulty locating the plane source before 
it comes within voice communication range. 

Range. The vacuum communication range be¬ 
tween this P-G unit and a 40-watt type W unit was 
determined in laboratory tests. The receiver per¬ 
formances of the two units are comparable (see 
“Laboratory Tests”), but the sources differ greatly 
in their efficiency, and consequently communication 
can be transmitted from the P-G unit to type W 
over a considerably greater range than in the oppo¬ 
site direction. This difference may be summarized 
as follows: 

Communication Vacuum Range ACW Range 

From P-G to type W 19,000 yards 7,600 yards 

From type W (with 10x10- 

degree beam) to P-G 5,500 yards 3,600 yards 

Calculations from these data indicate that the 
later 100-watt model of type W could be adapted 
to meet the request of Project Control AC-226.03 
for a range of 3 miles (5,200 yards) between these 
two systems, by adjusting the beam spread to about 
7x7 degrees. Then the last line of the above table 
would become: 

From new type W (with 

7x7-degree beam) to 

P-G 9,600 yards 5,200 yards 

If the P-G unit were used for communication to 
a similar unit in another plane, as has been sug¬ 
gested, the vacuum range in either direction would 
be about 23,000 yards and the ACW range about 
8,200 yards. 

For communication between the P-G unit and a 
shipboard type E unit, the vacuum range in either 
direction would be about 35,000 yards and the 
ACW range about 10,000 yards. 

Evaluation. The P-G system, if completed as 
originally designed, promises to have approximately 
the size, weight, range, beam angles, and security 
originally specified for it. No recommendations for 
changes can be made other than those already set 
down for the similar type E system (see “Evalua¬ 
tion” and “Recommendations,” Section 4.4.2). 






ELECTRICALLY MODULATED ARC SYSTEMS 


137 


Transmitter 

Source. The most efficient near infrared source 
available at the time the aircraft projects were 
begun was the 90-watt CL-2 cesium-vapor lamp 
which had been developed for the type E system. 
This lamp was too large to use in the space avail¬ 
able for the aircraft transceivers. Further, it seemed 
improbable that electronic equipment could be 
built for fully modulating any lamp consuming 
more than 50 watts of d-c power without exceeding 
the weight requirements. Accordingly, a smaller 
50-watt cesium vapor lamp was produced by West- 
inghouse Electric Corporation at the request of the 
Army Air Forces and Contract OEMsr-990. The 
lamp has a voltage drop of about 12 volts at the 
rated current of 4 amperes, and an intensity of 
about 75 hep measured on an RCA 919 phototube. 

Lamp Operation. The principles of operation of 
these lamps will be mentioned here, postponing to 
Section 4.4.4 the details of design of the three-lamp 
control panel as actually built. It was found that 
the lamps, once started, would operate successfully 
in series with a suitable ballast resistance directly 
from the aircraft 28-volt d-c supply. A small, com¬ 
mercial, current-regulating, iron-wire ballast re¬ 
sistor tube was used in the control panel to main¬ 
tain the lamp current at its rated value of 4 
amperes. 

Lamp Starting. To save weight in the aircraft 
installation, a simpler starting circuit is used for 
these lamps than for the type E cesium sources. 
To break down the arc gap, 400 volts a-c is applied 
to the lamp through a ballast resistor which limits 
the current to 0.5 amperes. After the lamp has been 
sufficiently warmed by this discharge, the a-c is 
removed and the 28-volt d-c simultaneously ap¬ 
plied, and the current slowly climbs to its normal 
operating value. 

Lamp Modulation. The output from the power 
amplifier is capacity-coupled directly to the termi¬ 
nals of the lamp as in type E. To avoid excessive 
dissipation of the modulator power in the low 
ballast resistor used here, a 4-millihenry choke is 
placed in series with this resistor. 

Full modulation of the lamp is obtained with an 
amplifier capable of supplying about 15 watts to a 
1.6-ohm resistive load. The modulator, as con¬ 
structed, employed negative feedback and gave 
28 watts output at 1,000 cycles with 2 per cent 


third-harmonic distortion; the excess power is 
ample to compensate for line losses. Except for 
the negative feedback and a high-pass filter which 
sharply attenuates frequencies below 900 cycles, the 
circuit is conventional. It is used here, for the same 
reason as in the type E system, to enable the upper 
voice frequencies which are important for intelli¬ 
gibility to be modulated in the arc at a higher level. 
Volume compression was not used because it com¬ 
plicates the circuits, produces distortion, and would 
give a high background noise between sentences as 
a result of the high ambient noise level in air¬ 
planes. 

Control Panel. As has been mentioned, no sepa¬ 
rate control panel was built for the plane-to-ground 
problem. The transceiver was operated, in tests, 
from one section of the three-lamp control panel 
which will be described in discussing the plane-to- 
plane system, Section 4.4.4. 

Receiver 

Preamplifier. The preamplifier is placed in the 
back of the transceiver head immediately adjacent 
to the cell. It consists of a single high-gain pentode 
stage followed by a cathode-follower to furnish an 
output signal of reasonable level and at the same 
time to give a low internal impedance. The TF cell 
load resistor is about 2 megohms; about 75 volts 
is applied across the cell and resistor. 

Amplifier. The principles of the amplifier will be 
described here although the control panel actually 
built will be described in Section 4.4.4. The ampli¬ 
fier is simpler than that of type E, since it does not 
require the narrow pass band for code reception. 
A high-pass filter is again used to reduce the noise 
below 900 cycles. A cutoff at high frequencies is not 
used (except for the decrease produced by a capaci¬ 
tor across the plate resistor of one tube), because 
it is felt that it would reduce intelligibility more 
than it would reduce noise. 

With the exception of the high-pass filter, the 
receiver-amplifier is a rather conventional high- 
gain audio amplifier employing negative feedback 
over the power output stage. Since at the limiting 
range of the equipment the input signal developed 
by the photocell is of the order of 2 microvolts, and 
the Army Air Forces requested that at least 200 
milliwatts be available to operate a pair of 600- 
ohm headsets, an overall voltage gain of about 140 
db was required. In the plane-to-plane system five 


fxpaqHiirTrR 



138 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


pairs of headsets were to be operated in parallel, 
so the amplifier was actually constructed to give a 
total available power output of 1 watt. The ampli¬ 
fier was designed using miniature tubes throughout, 
except for the power output tube. 

Optical System 

Directivity Patterns. The beam widths desired 
were very similar to those obtained with type E. 
The P-G optical system was therefore made a small 
replica of the type E optical system, but using the 
50-watt cesium lamp and the %x%-inch TF cell 
instead of the larger lamp and cell in type E. The 
reflector sizes and arrangement are indicated in 
Figure 25, T-5. 

The shapes of the resulting directivity patterns 
are similar to those shown for type E in Figures 13 
and 14, but with the widths and optics factors given 
above under “General Design.” The vacuum range 
of the P-G unit transmitting to another P-G unit 
is less than half that of type E transmitting to type 
E, because of the smaller source intensity and re¬ 
ceiver area and the larger transmitter beam width in 
the P-G system. 

The range of the P-G unit for an infrared electron 
telescope was given above under “Security.” 

Filter . The filter recommended for use with this 
transmitter is the Polaroid XRN5PX-55. The NVR 
has not been measured directly but was computed 
from the type E NVR measurements (see “Filters,” 
Section 4.4.2) with the results given above under 
“Security.” This filter is nearly perfect for the 
cesium lamp (see Chapter 2). Therefore, if any 
decrease of NVR of the P-G system is desired, it 
should be produced not by increasing the filter 
density, but by decreasing the lamp input power; 
either procedure results in a corresponding decrease 
in vacuum communication range, but the latter is 
more efficient. 

An OSU filter on white glass, with an ehT of 
0.5 or 0.6, should also be acceptable. 

Operational Tests 

Laboratory Tests. First laboratory tests with the 
P-G transceiver and the three-lamp plane-to-plane 
control panel showed the need of improved filtering 
inserted in the d-c line control panel to eliminate 
ripple pickup during reception, as described in 
“Operational Tests,” Section 4.4.4. 

Subsequent laboratory range tests between the 


P-G unit and type W indicated vacuum range 
values as given above under “Range.” A compara¬ 
tive study of the sensitivity of the P-G and type W 
receivers was made by finding the vacuum range 
to each with a bare 90-watt cesium lamp source 
operated from a type E transmitter. The ranges 
were, to type W, 5,600 yards; to P-G, 6,700 yards. 
The increased range of the P-G receiver is ac¬ 
counted for almost entirely by its larger reflector 
area, as the sensitivity of the TF-cell receiver cir¬ 
cuits is evidently about the same for both systems. 

Field Test. One test was made of actual commu¬ 
nication between the P-G transceiver and a com¬ 
mercial model type E system over distances between 
3 and 4 miles, on August 22, 1945, during the course 
of a ship-to-shore test of the latter on Lake Mich¬ 
igan. No attempt was made to determine maximum 
range. The quality of transmission from the P-G 
unit appeared superior to that from type E, the 
quality of reception on the two units about the 
same. 

Revisions. In the latter test it was found that 
generator ripple noise leaked into the receiver 
through electrostatic coupling to the photocell. This 
was cured by placing an electrostatic screen over 
the receiver mirror as shown in Figure 24. Further 
ripple noise came in as a result of backscattered 
light from the transmitter; subsequent laboratory 
tests indicated that the addition of a second section 
of L-C filter in the input d-c power lead eliminated 
the trouble. 

Test at Wright Field. The P-P control panel and 
power supply, a starting box, the P-G transceiver 
unit, the added d-c line filter just mentioned, and a 
type W ground unit (the third one built, which had 
a 40-watt lamp) were sent to the Special Projects 
Laboratory at Wright Field in September 1945 for 
operational tests between plane and ground. 

At this writing, one such test has been conducted 
and an informal report has been received from that 
laboratory. The test was carried out on a clear night 
with a bright moon and some ground haze. In trans¬ 
mitting from the plane (16 degrees hpi width) to 
the ground unit (10 degrees hpr width), about 85 
per cent* intelligibility was maintained to a distance 
of about 6 miles, with the plane at an altitude of 
5,500 feet. In transmitting from type W on a 12- 
degree beam to the plane unit (12-degree hpr 
width), intelligibility was reported at about 25 per 
cent at a distance of 1 mile with the plane at an 


R 



ELECTRICALLY MODULATED ARC SYSTEMS 


139 


TRANSMITTERS RECEIVERS 

PLANE-TO-GROUNO SYSTEM r-j2 




TAIL UNIT R-8 2 


SAME LAMP 

TWO BEAMS EACH 70° X 70° 
CENTEREO AT ±60° 
O t = 4X AT 50° AZIMUTH 
NVR =60 YD 


SAME CELL 

TWO BEAMS EACH 70° X 70° 
CENTERED AT +60° 
O r = 3X AT 50° AZIMUTH 


LOCATION AND DIRECTION OF LAMPS FOR PLANE-TO-PLANE COMMUNICATION 



NOTES 1 NVR FOR POLAROID FILTER XRN5PX-55 (ehT = 0.55) 

2 RECEIVERS USE SAME KIND OF MIRROR WITH SOURCE REPLACEO BY PHOTOCONDUCTIVE CELL 

Figure 25. Aircraft optical systems. 












































140 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


altitude of about 1,000 feet. A visual signal lamp 
was used to maintain contact at the larger ranges. 

The personnel of the Special Projects Laboratory 
was reported to be pleased with the general per¬ 
formance of the P-G unit but dissatisfied with that 
of the particular type W unit used, which employed 
a 40-watt source and wide beam. Various difficulties 
with the P-G unit were reported, such as the tedi¬ 
ousness of lamp starting, which may have been due 
to undervoltage on the aircraft generator. There 
was high noise at the plane receiver, both ambient 
and from backscatter. The latter is probably due to 
placing the unit too far inside the hatch door. It 
should be located within a few inches of the air- 
stream and well clear of the door frame. It was 
essential in avoiding backscatter in type E, for 
example, that no local objects be within about 
50 degrees of the axis of communication at any time. 

There was a great deal of trouble in maintaining 
contact between the units. This will undoubtedly 
disappear when the inferior charged-phosphor in¬ 
frared image tubes (metascopes) are replaced by 
the recommended C 4 electron telescopes. The beam 
and receiver angles of the units are very close to 
those specified from the early tests at Wright Field 
(see “Course of Development of Aircraft Systems,” 
above). If adequate telescopes are used there should 
be no more difficulty than those early tests indi¬ 
cated. 

Most of these troubles will undoubtedly decrease 
as the operating personnel become more familiar 
with the units. Especially, it seems likely that the 
range from type W will come more into line with 
the laboratory estimates (see “Range,” above) 
after the operators become expert in making and 
maintaining the very delicate grid adjustment in 
the laboratory model. Two of the improved 100- 
watt type W models have been sent to the Special 
Projects Laboratory for succeeding tests. 

It is of interest that in this first test the difficul¬ 
ties anticipated with noise in the receiver from the 
hot motor exhaust did not materialize. 

Present Status 

The units listed above have been left at Wright 
Field for further tests. It is expected that Navy 
Contract NObs-28373, which is continuing the work 
of Contract OEMsr-990, will be consulted in con¬ 
nection with any further development of the air¬ 
craft units. 


Recommendations 

The P-G unit appears to meet the specifications 
for which it was designed. Whether the type W 
ground unit needs further improvement and 
whether changes are necessary in the P-G unit can 
be decided only after more extensive operational 
tests. 

Probably mechanically modulated systems should 
also be built up and compared with the plane unit 
for aircraft use. They are less efficient and may be 
susceptible to vibration, but the modulator weight 
is negligible and the inefficiency may be counter¬ 
balanced by the use of very high-powered tungsten 
lamps with little increase in weight. A mechanically 
modulated system might be built to Air Force speci¬ 
fications with a weight of the order of 40 pounds. 
With a 1-kilowatt tungsten lamp source it might 
give ranges and angles comparable to those of this 
cesium lamp unit. 

4.4.4 Aircraft Systems: Plane-to-Plane 
[P-P] Communication System 

Description and Performance 

The specifications and course of development of 
the plane-to-plane system have been discussed in 
Section 4.4.3. The P-P installation on a single 
bomber was to consist of (1) three fixed trans¬ 
ceiver heads, each containing a source, photo¬ 
detector, optical systems, and a receiver-preampli¬ 
fier, (2) starting transformer boxes for the three 
sources, (3) a control panel containing the trans¬ 
mitting and receiving circuits, capable of modulat¬ 
ing one source at a time and receiving from one or 
all of the receivers at one time, and connected with 
the plane’s intercommunication system, and (4) a 
power supply which could be incorporated in the 
control panel. 24 

General Design. The location of and the direction 
of the axes of the three heads, two in the nose and 
one in the tail, is indicated in the sketch at the 
bottom of Figure 25. Sample heads were completed 
except for preamplifier circuit details connected 
with the choice of detector cell. The sources were 
the 50-watt cesium-vapor lamps (Chapter 1) and 
the photodetectors were to be either the large 
type B (2xl : *4-inch) TF cells or PbS cells of the 
same size or larger, if they proved better for day¬ 
light communication. The optical units were con- 



ELECTRICALLY MODULATED ARC SYSTEMS 


141 


structed to give the greatest intensity and response 
in the general direction of the adjacent planes in a 
normal flight formation, that is, at about ±50 
degrees azimuth, and zero degrees altitude, with 
respect to the flight axis. 

This distribution is obtained at the nose of the 
ship by pointing the unit on each side at 50 degrees 
away from the line of flight. The radiation source 
and detector cell are each placed axially in a 
6-inch diameter hexagonal cone, made of 6 plane 
mirrors as shown in the sketch under T-6 in Figure 
25. The beam from such a system is, of course, very 
uniform, the hpi and hpr fields both being close to 
60x60 degrees. The transmitter optics factor is 
about 4X 5 the receiver optics factor is less, about 
3X, because of the geometry of the cell. 

Of two tail transceiver models constructed, the 
best was a unit intended to be mounted high up in 
the vertical stabilizer of the ship. In this unit, 
transmitter and receiver each have two apertures, 
one on each side of the ship, about 6 inches square. 
Source and cell are each surrounded by five plane 
mirrors as shown under T-8 in Figure 25. The dis¬ 
tribution of intensity and response is expected to be 
in two beams, each about 70 degrees by 70 degrees 
square, centered at ±60 degrees azimuth from the 
line of flight. The transmitter optics factor at ±50 
degrees should be about 4X and the receiver 3X- 

The directivity pattern of this unit has zero 
range directly to the rear of the plane as far as ±25 
degrees away from the line of flight. The other tail 
unit which was built had its maximum range di¬ 
rectly to the rear but was much poorer at ±50 
degrees from the axis, in the direction of the ad¬ 
jacent airplanes. 

The nose and tail transceiver units must each 
have a starting transformer box near by. A single 
control panel in the center of the ship contains the 
other transmitting and receiving apparatus for the 
three transceiver units (Figure 26). 

The dimensions and weights of the units are: 


Front transceiver heads (8 lb 


each) 

Recommended tail transceiver 

8x8x14 in.each 

161b 

head 

8x10x13 in. each 

151b 

Starting boxes (10 lb each) 

4x6x8 in. each 

30 lb 

Control panel } , ,. , 

Power supply | may be combined 

8x10x19 in. each 
5x8x19 in.each 

24 lb 
171b 

Total weight 


102 lb 


The expected power consumption of the whole 
equipment would be about 350 watts from the 28- 


volt d-c plane supply plus an estimated 130 watts 
from the 115-volt 400-cycle a-c supply. 

Security. The filter used over each transmitter 
is the XRN5PX-55 PYA sandwich-type Polaroid 
filter; an equivalent OSU filter could be used. The 
NVR is about 110 yards for each transmitter unit 
as computed from the type E NVR measurements 
of Contract OEMsr-990 and about half this if 
determined by NRL methods. It would seem desir- 


Figure 26. Three-lamp P-P control panel and power 
supply. 

able to relax the specification of 40 yards NVR 
given in Project Control AC-226.04, if work on 
these systems is to be carried further. The filters 
are nearly perfect for the cesium lamp. Therefore 
any reduction below the present visual range will 
reduce the communication range, which already 
may be short of the desired minimum value. 

For a type C 3 or C 4 infrared electron telescope 
the estimated maximum vacuum range of detection 
for each of the transmitter units is about 16,000 
yards and the ACW range 7,000 yards. 

Estimated Range. Because of the unsolved prob¬ 
lems connected with the best use of photodetector 
cells in daylight operations, only order-of-magni- 
tude estimates of daylight range can be given. These 
estimates predicate the use of TF cells as photo¬ 
detectors; present information on sensitivity and 
behavior with background light of PbS cells is too 
scanty even for such estimates. 

The TF cell daylight range estimates are based 
on two sets of data: (1) night ranges computed 
for the P-P system from the lamp holocandle- 
power, cell sensitivity, and optics factors, the com¬ 
putations being based on experience with type E 








142 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


TF-cell receiver circuits; and (2) preliminary lab¬ 
oratory measurements with TF cells of the loss in 
sensitivity (S/N ratio) produced by background 
illumination roughly equivalent to average over¬ 
cast north sky light, the measurements being made 
with the receiver load resistor properly reduced to 
compensate for the reduced resistance of the cell 
under these conditions. In these measurements of 
background effects the cell was covered, as it should 
be in all daylight work, by a filter similar to that 
over the transmitter so as to reduce the background 
intensity as much as possible as compared with the 
signal. 

The first set of data, the computed night ranges 
for the P-P system, gives the following result: 
maximum vacuum voice range, at azimuth ±50 
degrees and altitude zero degrees, about 4,000 
yards, ACW range about 2,800 yards, both values 
being subject to uncertainties perhaps of the order 
of 30 per cent. For night communication between 
two airplanes flying parallel in the same horizontal 
plane, the variation of range with azimuth is shown 
in Figure 27A (one quadrant). The variation with 
altitude, when the line between the airplane is at 
50 degrees azimuth, is shown in Figure 27B (one 
quadrant). 

In the second set of data, the sensitivity loss of 
TF cells, with daylight background but under 
optimum conditions, was measured at about 20 db. 
With such a loss, the vacuum range of the system 
would be reduced by a factor of about 3, giving an 
estimated daylight vacuum voice range of about 
1,200 yards, ACW range of about 1,000 yards. 
These daylight range estimates are uncertain by 
perhaps a factor of 2 in either direction, and indeed 
the ranges might vary by this much or more from 
day to day, or from low to high altitude, or from 
one direction of flight to another. With present cir¬ 
cuits, unless the optimum daylight load resistor is 
changed for use at night, the night ranges will be 
only slightly greater than these values. However, 
a proposed new circuit (see “Preamplifier” in this 
section) may make it possible to obtain both opti¬ 
mum day performance and optimum night perform¬ 
ance with the same circuit without any changes. 

The PbS cell is less sensitive to background light 
than the TF cell but also has a much lower sensi¬ 
tivity in the dark. However, its sensitivity increases 
at low temperature and it might be placed so as to 
be cooled by the airstream and protected from the 


heat of the transmitter to take advantage of this 
property. Whether it would be better or worse than 
the TF cell for average daylight conditions cannot 
be predicted at present. It might operate in direct 
sunlight where a TF cell would be completely 
deadened. 


0 ° 30 ° 



90 ° 60 ° 30 ° 



Figure 27. Area of two-way night communication, 
P-P system (planes flying parallel). With (A) vac¬ 
uum night range as a function of azimuth and altitude 
0°, and (B) vacuum night range as a function of the 
altitude angle and azimuth 50°. 

The use of a streamlined shelf projecting out a 
few inches into the airstream to act as a sunshade 
over each of these receiver optical units might ma¬ 
terially increase daylight communication range for 
TF cells and still have little effect on the angle of 
view of the receivers. 

Evaluation. The system probably will not meet 
the specifications of an NVR of 40 yards, which, 
however, seems unnecessarily stringent. The ques- 









ELECTRICALLY MODULATED ARC SYSTEMS 


143 


tion of whether it will meet the requirement of 
Vk-niile daylight range cannot be settled except by 
operational tests and further work on receiver cir¬ 
cuit design and comparison of results with different 
types of detector cells. Whether, in the system as 
designed, the distribution of intensity and of re¬ 
sponse as functions of azimuth and altitude would 
prove satisfactory to the Air Forces is unknown. 

There are too many transceiver units in this de¬ 
sign and they are too far apart for efficient opera¬ 
tion (the B-29 fuselage length is approximately 
100 feet). A single unit on top of the tail or in a 
blister above the fuselage might give better angles 
of view and could be operated at considerably 
higher power for the same total weight. 

The estimated short range is a direct consequence 
of the wide angle of view desired. If it proves too 
short to be of military value it can be increased, 
with this angle of view, only by using much higher 
powered sources. Mechanical modulation of tung¬ 
sten sources of 1 kilowatt or over might give better 
range and require less weight, as mentioned under 
“Recommendations” in Section 4.4.3. 

Transmitter 

Lamp: Operation , Starting, Modulation. The 
lamps used and the principles of operation are the 
same as described for the P-G transmitter in Sec¬ 
tion 4.4.3. 

Control Panel. The basic modulation circuit is 
described under “Lamp Modulation” in Section 
4.4.3. To reduce weight, only a single high-pass 
wave filter was used. It was shifted by the send- 
receive switch from the modulator to the receiver 
as needed. 

The three receiver preamplifier outputs are am¬ 
plified through one stage and then mixed electroni¬ 
cally before going to the high-pass filter. The 
selector switch provides that any one or all of these 
three channels can be in use at any one time. Each 
channel is completely independent of all of the 
others. 

In send position, the output from the wave filter 
goes to the feedback section of the modulator power 
amplifier; in receive position, to a small two-stage 
power amplifier for operating from one to five sets 
of 600-ohm impedance headphones. 

The high-voltage power supply was constructed 
on a separate chassis to avoid hum leakage from 
the 400-cycle power transformer to low-level ampli¬ 


fier components, but this precaution proved un¬ 
necessary. 

Figure 26 shows the control panel and power sup¬ 
ply. The three ammeters read the lamp current of 
each transmitting channel. The voltmeter reads the 
d-c arc voltage drop of the lamp which is being 
modulated, or if the lamp is not running it indicates 
this fact by reading d-c line volts. The switch in 
the upper right-hand corner is the send-receive 
switch. On the left of the send-receive switch is the 
channel-selector switch. It has four positions: send- 
receive on channels I, II, or III only, or, in the 
fourth position, receive on all three channels. 

Receiver 

Photodetector Cell. The cells considered under 
Contract OEMsr-990 were the type B TF cells used 
in type E, and a PbS cell of the same size, con¬ 
structed under Contract OEMsr-235 for this prob¬ 
lem. 

Preamplifier. The basic preamplifier circuit is that 
described under “Preamplifier” in Section 4.4.3 for 
the P-G system. The difficulty with the TF cell in 
such a circuit is that its resistance may decrease 
by a factor of 50 in going from dark to daylight 
background. If a simple fixed load resistor is used 
in series with the cell as in the other aircraft sys¬ 
tem, maximum power output evidently cannot be 
obtained under both conditions. It is not feasible 
in aircraft applications to change the resistor to 
meet different background conditions. The best 
present solution is to use a resistor suitable for 
daylight operation; night range will then be slightly 
greater than day range but much less than the 
maximum night range obtainable with the opti¬ 
mum load resistor for night operation. 

There was no time to try one solution to this 
problem which seems promising. This is the use of 
an audio reactor, with an inductance of 10 henries 
or more and a resistance of 500 ohms or less, in 
place of the load resistor. With such a choke feed, 
the cell voltage would remain constant day or 
night and no switching arrangements would be 
necessary to provide optimum operating conditions. 
The choke may also introduce some correction for 
the decrease in cell response at higher audio fre¬ 
quencies. 

Even with optimum adjustment of resistance the 
sensitivity of the TF cell decreases with background 
light. Laboratory measurements made under the 



144 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


equivalent of 1,000 footcandles illumination from an 
overcast north sky showed in one cell a loss in S/N 
ratio of the order of 20 db as compared to the ratio 
for the cell in the dark. 

Circuits for the PbS cell would probably be 
basically the same as for the TF cell, but time was 
lacking for making detailed studies. 

Amplifier. The principles of the amplifier for the 
P-G system have already been described under 
“Amplifier” in Section 4.4.3, and details have been 
mentioned above under “Control Panel.” 

Optical Systems 

The first laboratory units of this equipment were 
to be designed to operate through existing windows 
in the B-29 bomber without any new construction 
of transparent blisters and were to be mounted in 
easily accessible places, such as the nose blister and 
tail gunner’s compartment. 

It appeared that the use of a single unit in the 
center of the nose would interfere with the opera¬ 
tion of the plane, so it was decided to have two nose 
units, one on each side, as described above under 
“General Design.” The size and arrangement of 
parts in these P-P nose units can be the same as in 
the P-G transceiver, except for the change in re¬ 
flectors. 

At first an attempt was made to design a tail 
transceiver unit for a space 8 inches deep, with 
both optical apertures fitting into a 3x11-inch open¬ 
ing, just over the tail window of the ship. This 
space was found to be too small for any optical 
systems giving the desired distribution. A unit was 
then built for use in the vertical stabilizer as 
described. 

The rear side of the box housing the latter trans¬ 
ceiver may reflect radiation from the airplane’s own 
motors into the detector cell. On this account, this 
surface may perhaps need to be painted dead black 
as the TF cell is fairly sensitive to low-temperature 
radiation and the PbS cell extremely so (Chapter 3). 
The pulsed exhaust radiation may cause a serious 
noise problem in the receiver in spite of the filtering 
out of the low-frequency components. (However, 
see “Test at Wright Field,” Section 4.4.3.) Whether 
the radiation from the exhausts of adjacent planes 
will also cause trouble is unknown, though it seems 
very likely with the PbS cell. 

All three transceivers should have filters over 
both the transmitters and receivers for daylight use 


with TF cells. Each unit should be mounted with 
the transmitter above and the receiver below and 
with a streamlined shelf projecting out into the 
airstream between the transmitter and receiver as 
a sunshade. The units should not be placed inside 
existing windows in such a way that the trans¬ 
mitter beam can reflect back from the window into 
the receiver. 

The estimated range curves shown in Figure 27 
have been computed from laboratory measure¬ 
ments of the directivity patterns of the units ac¬ 
tually built. 

Operational Tests 

Since the receiver cell and preamplifier questions 
had not yet been solved, at the time of termination 
of the experimental work of the contract caused by 
the end of the war, no operational tests with these 
transceivers were made. Rough range estimates 
have been given under “Estimated Range” above. 

The operation of the control panel was checked 
in the P-G “Operational Tests” (Section 4.4.3). 
In the first of these tests it was found that 400-cycle 
harmonic ripple frequencies leaking back from the 
laboratory aircraft generator into the 28-volt d-c 
line were causing a great deal of noise in the re¬ 
ceiver. This was cured by inserting a single-section 
L-C filter in the d-c power line and by changing 
to a double-section R-C filter in the d-c supply 
voltage for the photocell. The later outdoor field 
test showed the need of further filtering to avoid 
the noise from backscattered light. Laboratory tests 
indicated that a second section of L-C filter in the 
d-c line eliminated the trouble. The control panel 
then worked quite satisfactorily. 

Present Status 

The P-P control panel is at Wright Field for 
tests on the P-G system (see “Present Status,” 
Section 4.4.3). 

Recommendations 

The communication ranges, security, and daytime 
ranges of the P-P system promise to Jbe close to the 
desired values. Whether or not this particular sys¬ 
tem is of continuing military interest, an incentive 
for completing and testing it further should come 
from the large number of interesting associated 
problems on which it is important to get exact quan¬ 
titative data in considering military applications 



CARRIER-WAVE SYSTEMS 


145 


of the infrared for aircraft in general. Some of 
these problems are: 

Proper number and location of transceiver units 
on aircraft for best communication. 

Comparative weight, power, range, and stability 
of mechanically modulated versus electrically mod¬ 
ulated systems. 

Problems connected with day-and-night receiver 
operation (also important for ground infrared 
uses), such as optimum circuit design, measure¬ 
ment of actual daylight and sunlight effects, choice 
of cells, and filtering and shading of receivers 
against background light. 

Improvements and changes in performance of 
receivers at high altitudes as a result of cold and 
low background light. 

Effect of engine exhaust radiation on voice and 
other reception with different types of cells. 

Associated with the consideration of mechanically 
modulated systems and PbS cells on aircraft is the 
possible use of the intermediate infrared, with its 
greater security against enemy detection (Section 
4.8). , 

45 CARRIER-WAVE SYSTEMS 

4,5,1 Types of Systems 

Three different types of infrared communication 
systems have been devised in which audio ampli¬ 
tude-modulation may be superimposed on a higher 
frequency carrier-wave modulation of the radiation 
beam. In the type exemplified by the two systems 
to be discussed in Sections 4.5.2 and 4.5.3, a gas 
discharge source is electrically modulated at high 
frequencies. In another type, reported in Section 
4.6, the state of polarization of the beam is modu¬ 
lated by use of the photoelastic effect in a block 
of glass strained by a standing wave. In a third type 
(the Scofoni system), not developed by NDRC, 
the beam is spread into an intensity-modulated 
diffraction pattern by a “grating” of standing 
supersonic waves in a liquid. 

Such carrier-wave systems may make use of 
some advantages not possessed by audio-frequency 
systems. (1) By using receivers tuned to the radio 
frequency, communication may be carried on in 
many different carrier-frequency channels, just as 
in radio. (2) They can be made secure against re¬ 
ception by ordinary audio-frequency receivers 
simply by changing them over from amplitude- 


modulation to frequency-modulation, or by super¬ 
imposing the r-f on a steady d-c current.* 

A German system using r-f FM with a cadmium- 
compound detector cell was apparently produced 
during World War II, but details are not avail¬ 
able. The source may have been a gas discharge of 
the type described in Sections 4.5.2 and 4.5.3 or the 
infrared mercury lamp already mentioned in Sec¬ 
tion 4.4.1. 

Signal Corps Optiphone 

The Scofoni system mentioned above has been 
applied in the U. S. Army Signal Corps optiphone. 11 
This is a very narrow-beam (% degree) system, 
with a narrow-angle receiver, obviously for land 
use only, with ACW voice ranges of the order of 5 
miles at night and 3 miles in the daytime. It oper¬ 
ates as follows. As the amplitude of the standing 
supersonic waves in the liquid is varied by the mod¬ 
ulation, the effect of the diffraction grating formed 
by the waves changes, throwing more or less light 
out of the central beam into the first and second 
diffraction orders which lie within about 1 degree 
to the right and left. This transmitter is not secure 
against audio reception, but the narrow beams 
probably are secure enough anyway; only an audio 
receiver is used in the system. Since this system and 
the photoelastic shutter system use the same super¬ 
sonic modulation principle in quite opposite ways, 
some further remarks will be made about this sys¬ 
tem under “Comparison with Optiphone” in Sec¬ 
tion 4.6.2. 

4,5,2 V-M System 

One electrically modulated gas discharge carrier- 
wave communication system was studied by the 
V—M Corporation, Benton Harbor, Michigan, under 
Contract OEMsr-1460. 25 On account of the very 
preliminary state of the equipment as demon¬ 
strated, the much more complete development of 
other communication systems, and the status of the 
war, the development of this equipment was dis¬ 
continued after a few months. 

The source was a gas discharge tube, filled with 
krypton and xenon, which operated at about 500 
volts at a maximum continuous current of 50 mil- 
liamperes d-c and which could be modulated at 
frequencies up to 100 kilocycles. The portable, 
battery-operated receiver used a gas-filled cesium- 



146 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


surface phototube and a superheterodyne circuit. 
The system was demonstrated to NDRC and repre¬ 
sentatives of the Armed Services on July 6, 1945, 
in a darkened basement over a range of 50 feet. 
It was found that the source tubes were overloaded 
by voice modulation so code was used. 

Since a complete check of the transmitter oper¬ 
ating voltages and of the optical characteristics of 
the transmitter and receiver could not be made 
during the short period of the contract and since 
the gas discharge source used was in a very incom¬ 
plete state of development, the maximum operating 
range which might be achieved with the system is 
unknown. 

45 3 Touvet System 

A much more successful system was built under 
Contract OEMsr-1391 which was set up by request 
of BuShips under Project Control NS-243 to pro¬ 
vide facilities for Captain Guy Touvet of the 
French Navy to construct and test a radio-fre¬ 
quency system of his own design for voice and code 
communication. 26 Captain Touvet is an officer of the 
Societe duplications Seientifiques, affiliated with 
the Claude Neon interests, which built a similar 
apparatus in 1939 for the French Navy. As the 
inventions were not covered by United States pat¬ 
ents, he refused to disclose any details of the sys¬ 
tem being constructed until it was sold to the U. S. 
Government in September 1945. The description here 
is accordingly based only on his own statements, 
on observation of the equipment during demonstra¬ 
tions (especially during the type E reception de¬ 
scribed below), and on papers which he turned 
over to BuShips at the time of the sale. The ac¬ 
count here is necessarily fragmentary and may 
contain conflicting statements. 

French Prototype Model 

The prototype system was said to have been 
tested in Algiers with an observed range of 16 miles 
at night and about 2 miles in daylight. The 200- 
watt radiation source was a Pyrex tube filled with 
rare gas, made invisible by a cellophane filter 
which gave a beam 30 degrees wide when placed in 
a 60-centimeter diameter reflector. The modulating 
system required 2 kilowatts of power. Two types of 
detector tube were used: a resonant gas cell and a 


special design of tubular cell. The hpr receiver 
angle was 3 degrees. 

Description and Performance of Present System 

General Design. One transmitter and one receiver 
unit, of approximately the design given for the pro¬ 
totype model, were constructed under Contract 
OEMsr-1391. The transmitter consists of (1) a gas 
discharge source mounted in a large reflector, and 
(2) a control panel containing a power supply and 
voltage stabilizer, the r-f exciter and modulator, 
and an audio modulator, as well as code oscillators. 
The receiver consists of (3) an undisclosed kind of 
phototube mounted at the focus of a large lens, and 
(4) a control panel containing an r-f receiver with 
audio amplifier designed to feed a high-speed tele¬ 
graph recorder, a loudspeaker, or earphones. The 
transmitter control panel is about 18x24 inches in 
cross section and 6 feet high, weighing perhaps 450 
pounds. The receiver control panel is about 18x24 
inches by 3 feet high weighing perhaps 150 pounds. 
One transmitter mirror used was 24 inches in diam¬ 
eter and 10 inches in focal length; the receiyer lens 
was 13 inches in diameter and 18 inches in focal 
length. The hpi and hpr angles are about 25 degrees 
and 2 degrees respectively. The transmitter draws 
about 12 amperes from a 110-volt, 60-cycle supply, 
and is designed to modulate a much higher powered 
lamp than the 200-watt source actually used. 

The carrier frequency is of the order of 120 
kilocycles in the present system, and any one of six 
channels can be selected at the transmitter or re¬ 
ceiver end. The carrier may be amplitude-modu¬ 
lated by voice or code; or frequency-shift coding 
may be used for greater security. 

Security. Through the filters used, the visibility 
of the source is comparable to that of the type E 
cesium lamp source through XR3X41; this implies 
an NVR of near 100 yards. An NVR of 500 yards 
using Navy binoculars was claimed for the French 
prototype model. 

Also, the apparent brightness of the present sys¬ 
tem viewed by an infrared electron telescope is 
similar to that of type E, suggesting an ACW image 
tube range near 9 miles. Modulation of the source 
produces no apparent flickering in an image tube. 

All panels and the whole transmitter cabinet are 
fully shielded to prevent broadcasting of r-f elec¬ 
trical radiation. 

The amplitude-modulation of the carrier wave 



CARRIER-WAVE SYSTEMS 


147 


makes possible voice reception by a simple a-f re¬ 
ceiver, such as that of type E, to about the same 
range as the system’s own receiver. 

Range. The ACW range seems to be about 6.5 
sea miles, vacuum range about 30 miles, with an 
80-watt source input power such as that used in the 
outdoor tests. If the transmitter will modulate a 
500-watt source, and if such sources are available, 
the ACW range might increase to about 8.5 sea 
miles. 

Evaluation. This system can perhaps best be com¬ 
pared to type E. The present model has perhaps 
three times the weight and bulk and consumes 
almost twice the power of the type E laboratory 
model. With the source power for which it was de¬ 
signed it might give perhaps 2 miles more range 
than the latter. It has a larger transmitter beam 
angle than type E but a receiver angle of only 2 
or 3 degrees compared to the 18 degrees of type E. 
At present it has no more security than type E 
against reception of voice communication by other 
audio receivers. Such message security should be 
one of the main reasons for using an r-f system and 
was requested in Project Control NS-243. 

The detector is rather sensitive to background 
light and trouble may be encountered from back- 
scatter if a transmitter and receiver are used side 
by side. 

The multichannel feature of the present system 
might be very useful in convoys. Also it can be 
easily adapted to a “lock-in” tracking system 
guided on a steady code tone from the transmitter 
which would not interfere with simultaneous voice 
communication on a separate channel. 

Transmitter 

Source. The source is a long Pyrex tube about 
*4 inch in diameter shaped into two flat spirals on 
top of each other. They are 4 inches in diameter and 
1 inch apart. The tube is filled with xenon and pos¬ 
sibly small quantities of other gases at a low pres¬ 
sure (probably about 5 millimeters of mercury). 
The electrodes are oxide-coated tungsten spirals. 
The coiled discharge tube is enclosed in a flat box 
with a glass front. A mirror forms the back of the 
box, reflecting radiation back through the source to 
increase the candlepower. 

Since apparently only one of the sources brought 
from France was still functioning during the final 
tests, it was conserved by being operated at about 


40 per cent of its rated power of 200 watts. The 
voltage drop was reported to be 80 to 100 volts at 
r-f currents of 1 to 2 amperes. It is said that lamps 
of this kind can be built in sizes up to 1,000 
watts. 

One of the features claimed for this source is that 
its spiral shape gives it a self-inductance and a nat¬ 
ural r-f oscillating frequency which can be used to 
advantage in producing the modulation. 

The lamp has a pale bluish-white appearance 
when viewed without a filter. The radiation is said 
to consist of a many-line or banded spectrum, with 
no appreciable continuum, in the range from about 
0.7 p to perhaps as far as 3.0 p. Over 70 per cent 
of the input energy is claimed to be radiated in this 
region, most of it concentrated between 0.78 p and 
1.0 p in the present system. However, the type E 
comparisons given under “Type E Reception of 
Source” indicate that the holocandlepower of this 
source, when run at 80 watts, must be comparable 
to that of a 90-watt cesium CL-2 source. From this 
one would conclude that the claim of 70 per cent 
efficiency in the region from 0.8 p to 1.0 p is excessive 
and that the true hololuminous efficiency in this 
region is about 20 per cent, like that of the cesium 
lamp. 

The xenon lamp is said to be capable of electrical 
modulation up to very high frequencies, with car¬ 
rier frequencies as high as 200 megacycles used in 
laboratory tests. The distribution of intensity 
among the spectrum lines is said to change with 
frequency at frequencies over 1 megacycle, and the 
wavelength of maximum intensity can be displaced 
at high frequencies as much as 0.5 p. (Similar phe¬ 
nomena have been reported in the literature for 
other discharges.) 

The source is designed to be started by a voice- 
operated relay. After the lamp starts the starting 
circuit is cut off automatically. The source is main¬ 
tained by the r-f power even when not transmitting. 
With the present circuits it is apparently suscep¬ 
tible to being extinguished by overmodulation. 

The source is the most interesting feature of the 
Touvet system. Its modulation at high frequencies 
indicates a time lag comparable with that in micro¬ 
flash lamps. If an appreciable fraction of its radia¬ 
tion lies beyond 1.2 p, as claimed, it might be most 
valuable for intermediate infrared work, as dis¬ 
cussed in Section 4.8. 

Filters. The filters used on the system were ap- 



148 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


parently similar in composition and spectral trans¬ 
mission curve to cellophane-based Polaroid filters 
of type XR3X41 but did not have quite so sharp a 
cutoff or so high a peak transmission as the latter. 
The difference may be due simply to aging of the 
French filters (see Chapter 2). 

Optical System. The source has been used at the 
focus of a 24-inch diameter, 10-inch focal length, 
silver-backed glass mirror, and of a 36-inch diam¬ 
eter, 15-inch focal length, Stellite mirror. The beam 
width is about 25 to 30 degrees, depending on the 
system. 

Transmitter Circuits. The transmitter electronic 
equipment consists of six panel units: a high-fre¬ 
quency exciter [TrA], a high-frequency modulator 
for the source [TrB], an audio-modulator with a 
high-gain input channel for a crystal microphone 
[TrC], a high-voltage rectifier power supply capa¬ 
ble of delivering 1 kilovolt-ampere [TrD], a 2 
kilovolt-ampere line-voltage stabilizer [TrE], and 
a connection panel. 

The units appear to be of conventional design. 
Unit TrC contains a master oscillator feeding into 
an amplifier capable of delivering 40 watts of audio 
power. This is used for audio plate modulation of a 
stage in the exciter panel TrA, which in turn feeds 
two amplifier stages. The frequency response is 
flat to ±1 db from 200 to 6,000 cycles. A 600-cycle 
and a 1,700-cycle oscillator for c-w modulation are 
incorporated in the amplifier. Unit TrD is of the 
condenser input full-wave rectifier type. Unit TrE 
was designed for test and measurement purposes, 
and therefore provides for manual adjustment of 
supply voltages and output power. It also contains 
voice-controlled relay circuits for automatic oper¬ 
ation. 

Receiver 

Photodetector Cells. The resonant gas cell de¬ 
tector mentioned as being used with the original 
French equipment is said not to depend on selective 
absorption of the radiation. It is supposed to give 
twice the communication range obtained with a 
cesium phototube but is still in the developmental 
stage and was not used in the present equipment. 
The special design of tubular cell has been reported 
to be 20 centimeters long and 6 centimeters in diam¬ 
eter, with a cylindrical photosurface mounted along 
the cell axis; it is said to require 800 volts for oper¬ 
ation. The nature of the photosurface was not dis¬ 


closed, but it was said to be sensitive from the vis¬ 
ible to 5.0 \i. 

Apparently an ordinary silver-cesium vacuum 
phototube was used in the present equipment. The 
threshold for voice reception was computed to be 
about 5 X 10" 10 watt input signal, but the receiver 
appears to be no more sensitive than that of type E, 
whose threshold is several times this. The reception 
angle of 2 degrees reported for the 18-inch focal 
length lens corresponds to an effective photosurface 
area of about 0.5 square inch. 

Electronic Equipment. The receiver electronic 
equipment consists of four panel units, including 
an a-c and d-c power supply (ReA), an audio am¬ 
plifier (ReB) to feed a high-speed telegraph re¬ 
corder (if desired), a loudspeaker or earphones, 
and a connection panel; and the r-f receiver panel. 
Unit ReB delivers an output of 10 watts. A switch 
selects (1) one stage or (2) two stages of resistance- 
coupled amplification for telephony, or (3) a two- 
stage resonance choke circuit tuned to 600 cycles 
with a peak width of 80 cycles for telegraphy. In 
positions (1) and (2) the frequency response is flat 
to ±1 db from 80 to over 6,000 cycles. A vacuum- 
tube rectifier provides d-c impulses for a high-speed 
relay. 

Operational Tests 

Three types of operations were conducted: 

1. NDRC-Armed Services demonstrations over 
i/ 2 -mile range. 

2. Study of reception of signal by type E re¬ 
ceiver. 

3. Range tests. 

Demonstrations. With the source in the glass 
mirror, good intelligibility was obtained over a 
1,000-yard range from the roof of Northwestern 
Technological Institute to Grosse Pointe Light¬ 
house with the receiver lens stopped down to a 
3-inch square aperture. At the time of the official 
demonstration to representatives of NDRC, Army, 
and Navy on July 6, 1945, it was found that a small 
audio-frequency TF-cell receiver in the lighthouse 
picked up the voice communication very well al¬ 
though it had previously been understood that the 
Touvet system was to be secure against such re¬ 
ception. 

To determine the seriousness of this defect, the 



CARRIER-WAVE SYSTEMS 


149 


following tests were made with the Touvet source 
and the Touvet and type E receivers. The purpose 
of these tests was to determine definitely to what 
extent the Touvet transmitter could send voice or 
code without being received at all or without being 
received intelligibly by the type E audio-frequency 
receiver. 

Type E Reception of Intelligence from Touvet 
Source. A type E receiver (of 14-inch aperture, 
using a TF cell) was mounted beside the Touvet 
receiver in the lighthouse. A Wratten 87 filter (with 
ehT of 0.80) was used over the type E receiver 
throughout the tests; no filter was used on the 
Touvet receiver excepted as noted below. The 
Touvet source on the roof of the Institute was fil¬ 
tered, but used without any reflector. The tests were 
in three parts, aimed at (1) sending code to the 
Touvet receiver and no signal to type E, (2) send¬ 
ing code to the Touvet and a steady tone to type E, 
and (3) comparing voice reception on the two re¬ 
ceivers. 

From the results of the tests it appeared that to 
accomplish objectives (1) and (2), the Touvet car¬ 
rier frequency was being changed by the coding key 
from a value close to the superheterodyne fre¬ 
quency of the Touvet receiver to a value very dif¬ 
ferent. The results were as follows: In test (1) the 
code was not received by the type E receiver, ex¬ 
cept for faintly audible clicks as the key was 
opened or closed, which might have permitted train¬ 
ing the receiver on the bare source at distances up 
to a mile or on the complete transmitter up to sev¬ 
eral miles; in part (2) these clicks were not audible 
on the type E receiver, being drowned out by the 
steady tone superimposed on the carrier wave. 

In part (3) both receivers gave good intelligi¬ 
bility, the S/N ratios being about the same for 
both. The limit of intelligibility was obtained on 
the type E receiver with the receiver aperture 
stopped down to P /2 inches diameter. As this is 
about the minimum aperture for reception from a 
bare CL-2 cesium source at this distance, the equiv¬ 
alent holocandlepower of the two sources measured 
on a TF cell must be comparable. The correspond¬ 
ing test of minimum aperture was not made with 
the Touvet receiver because it “would have neces¬ 
sitated certain changes in the adjustment of the 
receiver.” 

A strong d-c background light placed near the 
source raised the noise level in the Touvet receiver 


considerably; interposing a double layer of filter 
over the Touvet detector cell reduced the noise 
somewhat. The noise level of the type E receiver 
was only slightly affected by the background 
light. 

The source was observed with a type C-3 infra¬ 
red electron telescope during these tests, with re¬ 
sults as reported above in “Security.” 

The conclusion from these three tests with the 
type E receiver is that the Touvet transmitter as 
at present constructed (1) probably has adequate 
system security against audio-receiver detection 
when unmodulated frequency-shift coding is used, 
(2) has no system security when modulated coding 
is used and no message security against audio re¬ 
ception when voice is used, and (3) has message 
security at all times, but no system security against 
image-tube detection. 

Range Tests. Three tests were made with the 
Touvet system operating over a range of 5.6 sea 
miles between the roof of Northwestern Techno¬ 
logical Institute and Montrose Beach in Chicago. 
The source was operated at 1.15 amperes r-f cur¬ 
rent in the Stellite mirror. (This mirror had a meas¬ 
ured reflection coefficient of about 0.5, which may 
have decreased the range somewhat.) In the worst 
weather experienced on these tests, with atmos¬ 
pheric transmission about 0.4 per sea mile, code 
reception was reported as very good and speech re¬ 
ception was at or below threshold, fading in and 
out. 

The mean vacuum voice range as indicated by the 
tests is of the order of 30 miles, and the ACW range 
for 100 per cent intelligible communication is about 
6.5 sea miles. 

Present Status 

The one source tube in working order was pur¬ 
chased from Captain Touvet by BuShips in Sep¬ 
tember 1945, and the one-way communication 
equipment (excluding the source tube) constructed 
under Contract OEMsr-1391 has been transferred 
by OSRD to BuShips. 

It is understood that the necessary components 
to provide two-way communication are to be com¬ 
pleted by BuShips and that the characteristics of 
the overall equipment, and especially the source, are 
to be further studied by Contract NObs-28373 with 
BuShips, which has superseded NDRC Contract 
OEMsr-990. 





150 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


Recommendations 

Because of the secretiveness and resistance to 
proper inquiry and supervision throughout the 
course of Contract OEMsr-1391, and because of the 
unsatisfactory condition of the equipment when it 
was transferred to BuShips, the first and most im¬ 
portant problem is to reconstruct correctly the 
operation of the system and restore it to working 
order. The pattern of secrecy maintained in this 
case between the personnel of a contract and the 
sponsoring organization sets an unfortunate prece¬ 
dent and has led to unsatisfactory consequences. 
Future civilian scientific groups such as OSRD 
should take steps to prevent such situations from 
arising. 

The xenon source has novel and unique proper¬ 
ties for near and perhaps intermediate infrared 
work and should be studied carefully. The receiver 
has too narrow an angle of view for manual oper¬ 
ation on shipboard. In its present form it would re¬ 
quire a stable table and training system to be of 
any value for ship communication; a larger re¬ 
ceiver angle could be obtained with no appreciable 
loss in range by use of different receiver optics and 
perhaps different phototubes. More precise infor¬ 
mation should be obtained on the other types of 
receiver cells described by Touvet. 

Alteration of the method of modulation as sug¬ 
gested under “Evaluation” would give more message 
security. 

46 POLARIZATION SYSTEMS 

Types of Systems 

Two systems have been devised in which the state 
of polarization of the radiation beam is varied by 
the modulation. Such modulation may be arranged 
to give negligible variation in the intensity of the 
emergent beam, so that an ordinary a-f receiver 
will receive no signal from the modulated source. 
In both systems this is accomplished by having the 
modulation vary only the distribution of energy 
between two coincident beams of complementary 
(incoherent) polarization which have the same total 
energy. 

In either system, one arrangement is to have the 
two beams plane-polarized at right angles in which 
case the signal is received by eliminating one beam 
with a plane-polarizing analyzing sheet over the 


receiver. Or the beams may be circularly polarized 
in opposite directions in which case even such a 
receiver will get no signal unless a quarter-wave 
plate in the correct orientation is added to convert 
the beams to perpendicular plane polarization be¬ 
fore they strike the analyzing sheet. 

This second arrangement is more secure because 
in the first arrangement considerable analysis of 
the plane polarized beams may take place from 
water or earth reflections or haze scattering, pro¬ 
ducing a large percentage of ordinary intensity 
modulation in the beam. Also, plane polarizing 
sheets are now commercially common optical de¬ 
vices and are likely to be tried over a receiver by 
an enemy attempting to “break” a secure system, 
while large quarter-wave sheets are not yet so com¬ 
mon or so easy to produce. 

Systems like these are very inefficient compared 
with non-polarizing systems having an equivalent 
percentage of intensity-modulation of the source. 
Even with perfect polarizing devices 50 per cent of 
the light is lost in the initial polarization to create 
the two beams (unless a beam-splitting device can 
be used), and actual infrared polarizing sheets for 
the wavelength region from 0.8 to 1.0 p transmit 
only about 35 per cent of the incident radiation. 
Further analyzer and reflection losses reduce the 
actual efficiency to less than 20 per cent of obtain¬ 
able intensity-modulation efficiency. 

One of the polarizing systems which has been 
devised uses simple audio-modulation of the polar¬ 
ization, while the other produces the polarization 
by a supersonic vibration which modulates the 
beam with an r-f carrier wave. 

Type L 

The first system, type L, began as a variant of 
type E and was studied under Navy contract by 
Baird Associates, Cambridge, Massachusetts. The 
principles will be presented only in outline form 
here as the study was not an NDRC project, al¬ 
though information on type E circuits and the use 
of the cesium source and TF cells was furnished to 
this company by Contract OEMsr-990. The two 
polarized beams were to be produced by two adja¬ 
cent type E cesium lamp optical systems. These 
were to be covered by complementary infrared po¬ 
larizing Polaroid sheets, plus infrared quarter-wave 
sheets for producing circular polarization, if de¬ 
sired. The total intensity was to be kept constant 



POLARIZATION SYSTEMS 


151 


by audio-modulating the two lamps in opposite 
phase, with special arrangements for keeping the 
amplitude of the intensity modulation of the two 
beams the same and for reducing second-harmonic 
distortion in the sources, which would give rise to 
overall intensity modulation. It was felt that inten¬ 
sity modulation could be kept within a maximum of 
the order of 5 per cent of the total beam intensity 
and that this amount would give a tolerably secure 
system. 

Recent information furnished by courtesy of Sec¬ 
tion 660E, BuShips, indicates that the cesium lamps 
in this system are to be replaced by concentrated- 
arc lamps to give a narrow-beam system known as 
type L. 

462 Photoelastic Shutter System f 

Description and Performance 

Course of Development. A second polarization 
system has been constructed by MIT Contract 
OEMsr-576, Project Control NS-187. 27 ’ 28 ’ 29 The 
polarization is modulated by the photoelastic effect 
in a block of glass strained by a standing super¬ 
sonic wave. This system is an outgrowth of earlier 
work by the personnel assigned to this contract in 
attempting to adapt the Kerr cell for use in a po¬ 
larization system. This work was not very success¬ 
ful because the amount of luminous or infrared 
flux controllable through the small aperture of feas¬ 
ible Kerr cells is quite limited. 

Work on the photoelastic shutter was started in 
August 1942 at the request of the Army Air Forces 
and was continued under Project Control NS-187 
for the development of a two-way system for Navy 
use. A complete two-way system was transferred to 
BuShips in the summer of 1944 to serve as the basis 
for the production of pilot commercial models. 

General Design. The general design of the system 
is shown in Figures 28, 29, and 30. It consists of 

(1) a transmitter mounted on a tripod, containing 

(2) a 75-watt r-f modulator for (3) the photoelastic 
glass shutter or modulator block, which is illumi¬ 
nated through a Polaroid polarizer and a filter by 
the collimated radiation from (4) a 250- to 600- 
watt sealed-beam landing light. For two-way com¬ 
munication, each station must also have (5) a 

f For list of symbols used in the equations of this 
section, see end of chapter. 


receiver, mounted on a tripod, containing (6) a 12- 
inch diameter collecting lens or mirror, which 
focuses light through an analyzing sheet onto (7) 
an electron multiplier tube, the output of which is 
detected by (8) an r-f receiver and audio amplifier, 
feeding either a loudspeaker or earphones. 

SHUTTER ASSEMBLY FILTER 



Figure 28. Photoelastic transmitter arrangement— 
“T-l Panda.” 


The transmitter shown in Figure 29 is not the 
final design, which was completely enclosed, but an 
intermediate stage which shows more clearly the 



Figure 29. Arrangement and support of modulator 
block. 


method of mounting and driving the glass shutter. 
Five types of landing lights can be used inter¬ 
changeably but the best results were obtained with 
7-inch diameter lamps, of 250 or 400 watts power. 



























152 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


When 600-watt lamps are used cooling of the filter 
and Polaroids is necessary. 

The hpi beam width is about 5x5 degrees; a well- 
annealed spread lens could be used over the photo¬ 
elastic block to increase the beam spread to about 
12x20 degrees. 



Figure 30. Photoelastic shutter system receiver. 


In the final model, the glass modulator block is 
6x6 inches in area and 2 inches thick and is made 
of optical crown glass. 

The supersonic wave is set up in the block by a 
set of 12 X-cut quartz crystals 1 inch square and 
Ys inch thick attached to the top or side of the 
block. The crystals, their silver foil electrodes, and 
the block are soldered together by a special tech¬ 
nique. The crystals are driven by a 75-watt radio 
transmitter at some resonance frequency which 
gives maximum light modulation. Many resonance 
frequencies are found for this system, spaced about 
10 kilocycles apart between 300 and 2,000 kilo¬ 
cycles. In the final model, frequencies near 400 
kilocycles were preferred, and crystals were cut 
whose natural frequency for longitudinal vibrations 
along the x direction was in this neighborhood. 

The modulation response drops by 50 per cent 
from 100 cycles to 2,000 cycles audio frequency. 
This is probably the result of the decreasing re¬ 
sponse of the shutter system to sidebands above and 


below the resonance frequency. To compensate for 
this effect, higher audio frequencies are pre-empha- 
sized in the transmitter so that the response is 
essentially uniform from 100 to 3,000 cycles. 

The polarizing arrangement employed in the 
range tests is the “stripped” shutter whose method 
of operation is outlined in Figure 32. In this ar¬ 
rangement the polarizer consists of a series of strips, 
each strip covering the space between two nodal 
lines in the glass block. The strips have their po¬ 
larization axes oriented at 45 degrees to the nodal 
lines, but the axes are alternately at -j-45 degrees 
and —45 degrees, each strip being oriented perpen¬ 
dicularly to its neighbors. 

It will be shown below, under “Stripped Shutter,” 
that such an arrangement produces an emergent 
beam from the block which changes from right to 
left circular polarization at just the r-f frequency, 
the total emergent intensity being constant at all 
times. The amplitude of the r-f variation in polari¬ 
zation is governed by the audio signal. This signal, 
therefore, not only cannot be received by any audio 
receiver but it cannot even be received by an r-f 
receiver if it is covered only by a plane polarizing 
sheet. 

The receiver instead must be covered by an infra¬ 
red quarter-wave plate followed by a plane polariz¬ 
ing sheet. The multiplier tube then receives an 
audio-amplitude-modulated r-f light signal which 
is converted to a-f and sent to headphones in the 
usual way. 

Code may be sent either by using a 500-cycle 
oscillator built into the transmitter, or simply by 
on-off interruption of the carrier wave, with super¬ 
heterodyne reception. 

The receiver in the final model differs somewhat 
from the intermediate stage shown in Figure 30. It 
consists of a Farnsworth 6-stage infrared-sensitive 
electron multiplier mounted behind an iris dia¬ 
phragm at the focus of a 12-inch diameter, 18-inch 
focal length lens. The circular analyzer sheets are 
placed over the lens. The multiplier output stage is 
part of an L-C circuit tuned to the r-f carrier fre¬ 
quency. The coil is inductively coupled to the an¬ 
tenna of a standard Navy type radio receiver. The 
hpr width of the receiver field is about 2 degrees. 

Early models of the transmitter and receiver 
weighed about 30 pounds each, exclusive of tripods; 
the final models are somewhat heavier. The power 
supply and modulator amplifier are in a separate 





POLARIZATION SYSTEMS 


153 


8xl0xl6-inch cabinet. The receiver was designed to 
operate either from 110-volt 60-cycle a-c supply, 
or from 6-volt battery Vibro-Pack units. 

Security. Polaroid filters of types XR3X-61 to 
XR3X-74 were used with the final units. These 
would give excessive NVR values, up to % mile, 
with the 600-watt landing lights, except for the ad¬ 
ditional filtering action of the Polaroid infrared 
polarizing sheets which reduce the NVR to less than 
300 feet for these lamps. 

The transmitter is electrically shielded to pre¬ 
vent radio reception at distances greater than 200 
feet. 

The transmitter can be observed by an image 
tube in the beam, but neither voice nor code mod¬ 
ulation can be detected by such a tube. No ranges 
have been measured for such detection, but very 
rough estimates based on image tube experience 
with type E indicate ACW image tube ranges up 
to perhaps 8 sea miles. 

The transmitter in its final form cannot be de¬ 
tected by any audio-frequency receiver. It can be 
detected only by a tuned r-f receiver which is cov¬ 
ered by a quarter-wave plate and plane analyzer. 
This system therefore has the greatest message and 
system security of any NIR system developed by 
NDRC. 

Range. Apparently no attenuation measurements 
were made during the field tests on the system, so 
the observations cannot easily be reduced to stand¬ 
ard conditions, but the consensus of results indi¬ 
cates an ACW range of about 4 miles, using a 450- 
watt lamp, an 8x20-degree transmitter beam, and 
a 2-degree receiver angle of view. This range corre¬ 
sponds to a vacuum range of about 12 miles. On 
one occasion, with the 450-watt lamp and a 5x5- 
degree hpi beam, good reception was obtained at 7 
miles range. 

Evaluation. The photoelastic shutter system is 
simple and ingenious and, as will be seen below, 
it can be easily changed into variant forms which 
are secure even from each other. Nevertheless the 
highest modulation efficiencies obtainable are esti¬ 
mated to be about 5 per cent, as compared for ex¬ 
ample with 25 per cent for even the cesium lamp 
polarization system (see Table 2). In fact, this 
shutter system has the lowest modulation efficiency, 
and therefore the lowest range for a given lamp 
power, of any of the systems discussed here. 

On the other hand, because of the simplicity of 


its modulation method, very little power is re¬ 
quired beyond the lamp power, and the range at¬ 
tained for a given total power is comparable to 
that of the other systems. Weight and bulk also 
appear to be favorable, as far as laboratory models 
give an indication on these matters; but perhaps 
the best feature of the system from a military point 
of view is that its large components—the landing 
lights, and the standard 75-watt transmitter and 
r-f receiver—are all standard units. These advan¬ 
tages, combined with its high security and with the 
possible simultaneous use of several r-f channels as 
mentioned in the beginning of Section 4.5, make the 
photoelastic system decidedly worth further mili¬ 
tary development. 

Transmitter 

The elucidation of the possible states of polari¬ 
zation of the light beam and the theoretical deter¬ 
mination of the audio-frequency variation of the 
mean transmitted flux as a function of the super¬ 
sonic intensity are both very pretty problems. 28 
Some of the simpler results will be outlined here. 

Comparison with Optiphone. In passing, it may 
be mentioned that from one point of view (which 
we will not adopt in what follows) the shutter 
works essentially on the same principle as the su¬ 
personic shutter in the Scofoni system used in the 
optiphone (Section 4.5.1). In both systems the 
supersonic strain produces a periodic spatial varia¬ 
tion of the optical properties of the medium, creat¬ 
ing a diffraction grating. The amplitude of this 
variation determines the intensity of light in the 
different orders of diffraction. 

In the Scofoni narrow-beam system, these orders 
are spatially separated. The photoelastic shutter 
system uses a wide beam in which they all overlap, 
but the various orders are differently polarized and 
may still be partially separated by an analyzer. 

Optimum Conditions. The variation in the state 
of the polarization is produced by the variation in 
the birefringence (double refraction) of the glass 
which is in turn the result of the varying super¬ 
sonic strain. There exist certain conditions for ob¬ 
taining maximum birefringence for a given input 
supersonic energy. 

First, the birefringence is greatest for a longitudi¬ 
nal supersonic wave. In such a wave, the optical 
axis of the strained glass is parallel to the direction 
of propagation and perpendicular to the nodal 




154 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


planes. For a maximum effect the polarizer must be 
oriented at 45 degrees to this axis, and the analyzer 
parallel or perpendicular to the polarizer. For uni¬ 
form behavior over the whole surface of the block 
the optical axes must be everywhere parallel, which 
means that the nodal planes must be parallel. This 
is practically possible only in rectangular or square 
blocks in which the supersonic wavelength is short 
compared to the dimensions of the block. 

The polarized light incident on the strained glass 
is divided into two components, one vibrating (at 
optical frequency) parallel to the optical axis, and 
the other perpendicular to it. There is a difference 
Ap in the refractive indices of the glass for the two 
components and as a result one lags behind the 
other by a phase shift <f> after passing through the 
thickness t. 

4>=2x ^t, ( 11 ) 

where X is the wavelength of the light. If the ana¬ 
lyzer is crossed with the polarizer, the intensity 
transmitted through it is then 

/ = 7(i sin 2 ^. (12) 

In this system, X is limited to the near infrared. 
The thickness t used in tests was % inch to 2 
inches, being limited by the transmitter energy and 
quartz crystal sizes available and by the nonuni¬ 
formity of larger blocks. Some tests, which appeared 
rather promising, were made with one surface of 
the glass block silvered so that the light was re¬ 
flected back through it and the effective thickness 
was doubled. 

The value of Ap is proportional to the difference 
(p — q ) of Neumann’s photoelastic constants, 
which depends on the material of the block. Plastics 
have the largest difference but give excessive damp¬ 
ing. Various glasses were tried and were all found 
to be very satisfactory except Pyrex. The principal 
requirement for the glass is that the index of re¬ 
fraction be low, preferably with n D below 1.55. 
Fused quartz would be excellent, because of its low 
index, large value of (p — q ), and small damping, 
except that the damping is so small that the reso¬ 
nance points are extremely sharp and so there is no 
response to the voice-modulated sidebands! The 
power requirement for ordinary glasses is about 
0.1 watt per cubic inch for shutter operation but 
that for quartz is 10 to 50 times lower. A suitably 


uniform block of fused quartz prepared by the 
General Electric Company proved excellent for 
code transmission. It is thought that external me¬ 
chanical damping could be used to improve the 
voice response of quartz blocks while still keeping 
an extremely low transmitter input power. The 
value of Ap is also proportional to the quantity a/l, 
where a is the supersonic amplitude and l is the 
supersonic wavelength in the glass. These factors 
are determined by the input energy and by the kind 
of quartz driving crystals which are suitable. 

The input energy may not be increased indefi¬ 
nitely but instead should be carefully limited for 
optimum performance. The maximum possible in¬ 
tensity change at any point is produced by varying 
<f> from 0 to 180 degrees. The maximum time-aver¬ 
age intensity over the whole block is obtained if 
certain points have higher values of <f> (up to 300 
degrees) for limited intervals, but any increase of 
supersonic amplitude beyond this point causes a 
decrease of transmitted light intensity. 

This optimum energy corresponds to supersonic 
amplitudes of vibration of the order of 10" 5 to 10" 6 
centimeter in the glass, which is of an order of mag¬ 
nitude frequently attained in supersonic studies. 
However, normal operation is best with even lower 
amplitudes because of strong nonlinearity with high 
phase-shifts and because of excessive heating of the 
block, which introduces spurious birefringence. 

Uniform Shutter. The simplest case is that of a 
polarizer which is uniform over the whole surface 
of the block (hereafter called a “uniform shutter”) 
as indicated in Figures 31 and 32. The polarization 
of the light passing through the nodal planes 
(planes of no strain) is unchanged by the block. 
The emergent radiation which has passed through 
the antinodes is polarized elliptically, rotating in 
one direction through a region of expansion and in 
the other through a region of compression. If the 
phase shift is 90 degrees the ellipses become circu¬ 
lar; if it is greater the long axes become perpendicu¬ 
lar to the original orientation and, when the shift 
becomes 180 degrees, the light becomes linearly po¬ 
larized in this perpendicular direction. 

The light beams passing through adjacent anti- 
nodes are far enough apart spatially that they are 
incoherent, that is, the phase relations between 
them are not fixed but are changing randomly. The 
addition of the incoherent ellipses of opposite di¬ 
rections of rotation is equivalent to the addition 



POLARIZATION SYSTEMS 


155 


of two incoherent linearly polarized beams, one 
polarized along the major axis of the ellipses, one 
along the minor. The effect of the supersonic strain 
is then to change the distribution of energy between 
these two linearly polarized beams (A and B in 
Figure 32). 



Figure 31. Appearance of vibrating block between 
crossed polarizers. 


In fact not only plane-polarized light but incident 
light of any uniform elliptical polarization with 
these major and minor axes may be used. It is con¬ 
verted by the strained shutter into one beam of 
emergent light of exactly the same ellipticity, but 
smaller intensity, and a second incoherent beam 
of complementary ellipticity. 

The intensity in beam B is zero at time t = 0 
when the strain is zero; it increases in each half 
of the strain wave, whether the strain is positive or 
negative. From this result we can draw three im¬ 
portant conclusions. First, the r-f frequency in the 
emergent beam is double the supersonic frequency 
and an r-f receiver must therefore be tuned to the 
double frequency. Second, the average intensity in 
beam B depends on the peak supersonic intensity so 


that if the latter is audio-modulated a strong audio 
component will be present in the emergent beam. 
(This component tvas used in early range tests.) 
Third, the instantaneous intensity in beam B is an 
even function of the strain, and therefore is non¬ 
linear and gives only very small effects for small 
strains. 

The phase shift as a function of position on the 
block is evidently <f> m ' cos 6 where <f> m ' is the maxi¬ 
mum optical phase shift at the antinodes and 6 is 
the phase angle in the supersonic wave. Using 
equation (12) we may find the instantaneous aver¬ 
age transmitted intensity in beam B per unit area 
as a function of <£,/. It is only necessary to integrate 
from 6 — 0 to 6 = jt/2, because of symmetry. 
n _ n 

cos 6 J dO 

= ^[1 - M + m ')], ( 13 ) 

where J 0 is the Bessel function of order zero. Here 
I 0 is the maximum transmission through the Po- 
laroids when they are parallel—about 20 per cent of 
the incident intensity, for existing infrared Polar- 
oids, as noted above. 

The intensity in beam B, averaged over an r-f 
cycle, is then 




Here (f> m is the peak optical phase shift reached at 
an antinode during the cycle. The variable of inte¬ 
gration a is the phase angle of the time variation in 
the supersonic wave. 

From this equation, at a value of <£ m of about 
300 degrees, I t reaches a peak value of about half 
7 0 . Thus in audio modulation the maximum pos¬ 
sible crest-to-trough value of the audio component 
is about 10 per cent of the intensity of the beam 
before polarization. If the modulation is restricted 
to the linear part of the curve represented by the 
last equation the fraction is less. In practice, be¬ 
cause of the introduction of nonuniform stresses 
from heating, because of the frequent appearance of 










156 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 



STRIPPED SHUTTER 



STRAIN 


POLARIZATION 


0 



0/0 


0 


NODE 


NODE 


NODE 


NODE 


NODE 


NODE 



0/0 

<-<-<- 


TOTAL EMERGENT POLARIZATION 
TWO INCOHERENT BEAMS 


/ o° 

• \ • \ •' Q Q 


AFTER TRANSMISSION 
THROUGH */4 PLATE AT RECEIVER 


Q 




/ 

\ \ 


Figure 32. Polarization from photoelastic shutter. 








































POLARIZATION SYSTEMS 


157 


a second set of dark nodal planes perpendicular to 
the primary set (see Figure 31), and because the 
light may pass through the shutter at angles up to 
10 degrees from the normal, the maximum value 
reported has been about 5 per cent. 

An interesting method was worked out for evalu¬ 
ating the optical phase shifts attained with a given 
supersonic input. These were determined by “un¬ 
crossing” two Polaroids (with the supersonic en¬ 
ergy turned off) by an angle until the transmitted 
intensity just matched the average intensity through 
exactly crossed shutters with the given input. The 
value of I t is 7 0 sin 2 /l, and qp m can then be found 
from equation (14). 

The attainable values of light-current modula¬ 
tion ratio are near 1.5 on the linear part of the 
curve, for low-frequency audio variations, as is 
predicted by equation (14). Because of the drop in 
response away from the resonance point, the values 
fall with increasing frequency of audio modulation, 
from about 0.8 at 100 to about 0.3 at 2,000 cycles 
per second. 

The value of I t in equation (14) is approximately 
the amplitude of the double-frequency r-f compo¬ 
nent of the light. 

Stripped Shutter. Because of the drawbacks of 
the uniform shutter which were already mentioned 
—small output at low amplitudes, plane polariza¬ 
tion, and audio modulation—an exhaustive exam¬ 
ination was made of other methods of utilizing the 
double refraction. The most satisfactory is the 
stripped shutter, indicated in Figure 32. In this 
arrangement, alternate antinodes of the supersonic 
wave are covered by complementary Polaroid strips, 
oriented first at +45 degrees, then at —45 degrees, 
then at +45 degrees, and so on. For this case the 
polarization ellipses all have the same direction of 
rotation at a given instant, but half their major 
axes are at +45 degrees, half at —45 degrees. The 
total resultant beam is equivalent to two incoherent 
circularly polarized beams ( A ' and B' of Figure 
32). A is more intense than B in one half cycle, less 
intense in the other. At t = 0, when the strain is 
zero, the light is unpolarized, that is, A and B are 
of equal intensity. When a quarter-wave plate is 
placed over the receiver, these circular beams are 
converted to the plane polarized beams A" and B", 
one of which is then transmitted through the plane 
analyzer to the receiver. 

With the stripped shutter the beam reaching the 


receiver is then stronger in one half cycle and 
weaker in the other. In this case, then, the received 
r-f frequency is equal to the supersonic frequency; 
the average intensity is independent of the super¬ 
sonic amplitude, and no audio modulation is de¬ 
tectable; and finally, the intensity is linear in the 
strain and therefore is larger than with the uniform 
shutter for small strains. This method has all the 
advantages the other one lacks. Because of the 
linear feature, much less driving energy (in experi¬ 
ments, only % as much) is needed to obtain the 
same signal at the receiver with ordinary signals; 
however, the uniform shutter theoretically becomes 
superior with very high signals. 




Figure 33. Comparison of analyzed modulation from 
stripped and uniform shutter. 


This stripped shutter is extremely secret, as it has 
the advantages of both circular polarization and 
absence of audio modulation. The kinds of signal 
produced by the two types of shutter are compared 
in Figure 33. 

The numerical expression of the intensity in A " 
and B” as a function of the supersonic amplitude 
has been evaluated only as a power series 28 and 
will not be given here. 

One other variant method of modulation has been 
proposed. This consists of the use of a steady super- 


^RFwrittrrrn 




















158 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


sonic carrier which is audio modulated by mechan¬ 
ical vibration of the stripped shutter. In this 
method, the boundaries between the strips are not 
placed at the nodes but at the antinodes. The signal 
produced is similar in character to that produced 
by the ordinary stripped shutter method outlined 
above, but the mechanical method would not be as 
efficient. 

Glass Blocks. In early laboratory models glass 
blocks 1/2 inch thick and l^xl^ inches in cross 
section were used. An increase of area was made 
necessary by the deterioration of the strongly ab¬ 
sorbing NIR polarizing and filter sheets under 
strong radiation intensity. By distributing the light 
flux over a larger area and by employing air cool¬ 
ing from a motor blower it was possible to secure 
continuous operation with 600-watt light sources. 
The use of larger shutters also greatly simplified the 
optical system and permitted the use of the stand¬ 
ard landing lights. 

Increase of the thickness of the shutter improves 
its efficiency. A doubling of the optical effects can 
be effected either by doubling the thickness of the 
block or by doubling the amplitude of vibration. 
The latter requires doubling of the voltage on the 
driver crystals and hence increases the power by a 
factor of 4; doubling the thickness only doubles the 
power requirement. The use of thicker blocks makes 
it possible to operate at lower voltages and thus 
avoid corona and the danger of electrical break¬ 
down in the crystals. A drawback in using thicker 
plates is that thick commercial glass plate is not 
sufficiently strain-free for this purpose and the 
shutter must be made of well-annealed optical glass. 

The blocks are ground and polished with oppo¬ 
site sides parallel within 0.001 inch. The photo¬ 
elastic behavior of one block can be reproduced 
quite accurately by another block of identical 
dimensions. 

Driving Crystals. The several X-cut crystals 
attached to a given shutter are of course matched. 
All crystals are attached with the directions of their 
polar x axes in the same orientation (that is, per¬ 
pendicular to the face on which the crystals are 
soldered); the y axes are 90 degrees apart in 
neighboring crystals. The mosaic covers a small 
side of the shutter, thus for a 6x6x2-inch block, 12 
crystals of lxl-inch area are used in two rows of 
6 crystals each. 

The shutter operates best at a frequency close to 


the resonance point of the longitudinal vibrations 
of the driving crystals in the x direction, or close 
to an odd harmonic of this frequency. When sol¬ 
dered together the system of glass and crystals has 
a new set of resonance points which are much 
closer together than those of the crystals alone. 
Several active frequencies (that is, frequencies use¬ 
ful for the photoelastic work) of the shutter can be 
found near each frequency of the crystals. In fact, 
the shutter can be operated at a very large number 
of carrier frequencies distributed over a large range 
from about 100 kilocycles to 100 megacycles, but 
those near a resonance point of the crystals are 
easier to excite. 

The driving potential of the crystals is 1,000 to 
2,000 volts, requiring a thick crystal, about % inch, 
and consequently a fundamental frequency below 
about 3 megacycles, in order to avoid corona break¬ 
down. In order to have several nodal planes in the 
glass block, so that they will be straight and 
parallel, frequencies above 0.3 megacycles are neces¬ 
sary. However, observations have been made with 
frequencies from 100 kilocycles to 15 megacycles. 

In early tests a frequency of nearly 2 megacycles 
was used, but later, with thicker glass blocks, a 
frequency near 900 kilocycles was used so that the 
nodes would be separated farther. The radiation 
beam passing through the shutter then does not 
have to be so closely parallel for effective operation 
as it does when the nodes are close together. When 
the stripped shutter was used, it was found that 
even at this frequency too much light was crossing 
from one antinode region to the next in traversing 
the block, with a resultant serious loss of modula¬ 
tion efficiency. Consequently, the nodes were sepa¬ 
rated still farther in the final model by use of a 
still lower frequency, near 400 kilocycles. 

The energy transmission from the crystal to the 
glass is considered to have been rather inefficient 
in these models. If this could be improved, lower 
powered transmitters would be adequate for com¬ 
plete modulation. 

Soldering Quartz and Glass. No satisfactory 
cement for joining the quartz and glass was found. 
Many cements were tried but all were heated by the 
high frequencies and set up thermal strains; some 
failed to transmit the sound waves after a time. 

Soldering the crystals to the glass was therefore 
tried and proved quite successful, although it is a 
delicate operation. The process worked out 29a ap- 



POLARIZATION SYSTEMS 


159 


pears to be fairly novel and valuable and an outline 
of it will be given here; the interested reader should 
consult the original report for details. 

First, the crystal and glass are given a thick coat 
of silver by evaporation in high vacuum. Then they 
are placed together with a 0.001-inch layer of rose 
metal between, and heated judiciously by a hot 
plate until fusion occurs. A glycerine solution of 
tartaric acid at the proper concentration is used as 
flux. The silvered surfaces act as electrodes for the 
crystals and leads may be soldered on with a sol¬ 
dering iron. 

The copperplating process recently developed by 
Corning Glass Works might be a substitute for the 
silverplating in this method. 

Oscillator. The driving oscillator is a single-tube 
circuit, operating normally at 1,500 volts with 
75-watt plate dissipation. The constants of the tank 
circuit are carefully matched to give the maximum 
potential and energy transfer to the crystals which 
are connected in parallel with the tuning condenser. 
The audio modulation is effected by screen grid 
modulation. 

A special feature of this oscillator is frequency 
control by acoustic feedback. A single quartz crys¬ 
tal is attached to the shutter on the narrow side 
opposite to the driving crystals. Vibrations in the 
glass are transmitted to this crystal and create an 
emf which is amplified and controls the grid of the 
oscillator tube. Thus the shutter automatically con¬ 
trols the frequency of the oscillator, and the driving 
frequency follows the small drifts of the mechanical 
resonance frequency of the shutter occurring during 
continuous operation as the result of temperature 
changes. 

In the final models, the driving oscillator is housed 
in a small cabinet attached below the optical unit. 

Monitor. For control of the transmission at the 
sender, a small fraction of the light is intercepted 
by a mirror, % xl /2 inches in area, which sends the 
light to a photocell and amplifier. An output meter 
and telephone serve to record the intensity and 
quality of the light transmission. The circuit of 
this monitor is analogous to that used for audio 
reception in the receiver. This monitor unit is also 
mounted in the cabinet below the optical unit. 

Receiver 

The receiver consists of three parts: the ^optical 
system, in which the light is collected by a photo¬ 


detector cell, the radio receiver, and a voltage 
supply for the detector cell. The latter two are in 
a separate cabinet and are of standard design and 
construction. 

In early tests the optical system consisted of an 
8-inch diameter concave mirror which focused 
light on an infrared-sensitive gas-filled phototube 
which was suitable for use with the uniform 
shutter and audio reception being used at that 
time. 

Later, for r-f reception, a Farnsworth 6-stage 
infrared-sensitive electron multiplier was used. A 
12-inch diameter, 18-inch focal length glass lens 
was used as the collector, because the field could 
then be stopped down with an iris diaphragm to 
improve daylight communication. 

An optical system with such a low //number 
necessarily has a narrow angle of view, here about 
2 degrees. The width of this angle could be approxi¬ 
mately tripled, with no loss in sensitivity, by return¬ 
ing to a mirror system; the narrow-angle system is 
valuable only for daytime work. 

Variable Quarter-Wave Plate. One of the most 
interesting and scientifically valuable results from 
Contract OEMsr-576 was the discovery of methods 
of duplicating the action of a quarter-wave polariz¬ 
ing plate by using two or three plates having phase 
differences different from a quarter wave. 28 Two 
such plates, for which the phase differences are any¬ 
where between one-eighth wave and three-eighths 
wave, when placed together in proper orientation, 
will convert a particular orientation of plane 
polarized light into circularly polarized light. Thus 
two plates which are each quarter-wave plates at 
just 0.6 p may be used together to serve as a 
quarter-wave plate for any wavelength between 
0.4 p and 1.2 p. Such combinations were used by this 
contract over the receiver in early studies before 
infrared quarter-wave plates became available. 

The combination of two phase-difference plates 
does not behave identically like a quarter-wave 
plate for light of all orentiations; but a combina¬ 
tion of three arbitrary phase-difference plates may 
be made to do so. 

Operational Tests 

All field tests were carried out between fixed 
stations, one unit being in the tower of the Blue 
Hill Observatory and the other at one of five receiv¬ 
ing stations at distances of %, 1%, 2%, and 7% 



160 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


miles away. Tests for two-way communication 
could be made only over the 2 1 /8-mile range because 
none of the other stations could be supplied with 
a-c power for the transmitter. From the S/N ratios 
observed on a given test at a given station, the 
threshold ranges of the equipment used could be 
estimated. Unfortunately, no attenuation estimates 
seem to have been made on these tests, and the 
reduction of the results to ACW or vacuum condi¬ 
tions is very uncertain. 

Part of the light path to all stations is over water, 
and the resulting differential thermal convection 
currents were held to be responsible for excessive 
“twinkling” and loss of intelligibility observed in 
some tests on clear quiet nights. 

Altogether, four separate transmitting and re¬ 
ceiving units were constructed. Following an early 
system which had 1%-inch square modulator blocks, 
cemented crystals, and audio transmission, a second 
system was built. This had a soldered shutter of 
4x6x% inches, driven by four crystals at 920 kilo¬ 
cycles from a standard 75-watt Hallicrafter radio 
transmitter the tank coil of which was inductively 
coupled to an L-C circuit in which the crystals 
formed the capacity. Field tests in March 1943 
showed satisfactory voice communication with this 
system over a range of 1% miles, with an 8-inch 
receiver mirror, a multiplier phototube, and a two- 
stage amplifier. 

Early Demonstration. The first instrument to 
have most of the features of the final unit was 
demonstrated to representatives of the Armed Serv¬ 
ices and of NDRC on May 10, 1943. The trans¬ 
mitter used a 250-watt landing light, a 6x6x1-inch 
shutter oscillated at 912 kilocycles with feedback 
control, and a 12-inch receiver mirror, a multiplier 
phototube, and a 4-stage audio amplifier. The hpi 
and hpr widths were about 5 degrees. Satisfactory 
daylight transmission with XR3X filter was ob¬ 
tained at 1% miles, while night transmission with 
XR7X filter was demonstrated at 2% miles. Under 
favorable weather conditions good voice communi¬ 
cation was also obtained with this system over the 
3% mile range. This transmitter has been operated 
for some 2,000 hours over 18 months without alter¬ 
ations of the electrical equipment or change of the 
soldered shutter characteristics. Some soldered-on 
shutters have been operated continuously at full 
power for 12 hours without any decrease of trans¬ 
mission. 


Final Units. Later this unit was modified for use 
with a 6x6x2-inch stripped shutter for 400 kilo¬ 
cycles single-frequency r-f transmission with a 
spread lens for 12xl2-degree beam spread. The re¬ 
ceiver was altered to have a circular analyzer, 
1-inch lens, and a radio receiver. 

A final and rather elaborate new system, con¬ 
taining several auxiliary features to facilitate test 
and measurement, was completed in November 
1943. This provides for quick interchange of com¬ 
ponents such as lamps, shutters, filters, and lenses, 
but is otherwise much as described above under 
“General Design.” A variable and a 500-cycle 
audio-frequency oscillator were incorporated in the 
transmitter. Tests with this system formed the basis 
for the redesign of the previous system just men¬ 
tioned. 

The S/N ratio was found to be increased seven¬ 
fold by r-f reception compared to audio reception. 
For code, on-off interruption of the carrier wave 
gave much greater ranges than did low-frequency 
code modulation of a continuous carrier wave. 

Tests showed that the use of a stripped quarter- 
wave plate is practicable and gives the same re¬ 
sults as the use of stripped plane-polarizers, as was 
expected from theoretical considerations. 

With a 250-watt lamp, 12x20-degree hpi beam, 
and XR3X-60 filter, a 4-mile range is attained in 
all but the worst weather. With a 450-watt lamp, 
and 5x5-degree beam, a 7-mile range was attained 
on one occasion; at other times only slowly spoken 
words were intelligible at this distance. 

Present Status 

The two final systems just described constitute 
sufficient equipment for two-way voice communi¬ 
cation. Upon request of BuShips these two systems 
were turned over to that group in July 1944 to serve 
as the basis for the production of pilot commercial 
models. Such pilot models were subsequently built 
by White Research Associates, Cambridge, Massa¬ 
chusetts. 

Recommendations 

This photoelastic shutter system is now well past 
the laboratory stage and requires only tests against 
other systems to determine its comparative military 
value. The performance of the shutters has now 
been carried to nearly the maximum which is pos- 


p urTR WI? Pfc 
H IU I ■ VI./ 




PROPOSED SPECTRAL MODULATION SYSTEM 


161 


sible with present materials, according to a rather 
completely developed optical theory which is itself 
of great interest. The modulation method per se is 
inherently inefficient, but the total power consump¬ 
tion appears not to be excessive compared to other 
NIR systems of similar range. The system is in¬ 
genious and simple and has the great advantages 
of good secrecy, choice of r-f channels, and the use 
of standard lamps, radio transmitters, and radio 
receivers. 

Better NIR polarizing sheets would be desirable 
for such systems. The use of fused-quartz shutter 
blocks deserves further study. A wider receiver 
angle would probably add to the military value of 
the system. 

47 PROPOSED SPECTRAL MODULATION 
SYSTEM 

Very recently another voice modulation system 
has been proposed 18b based on the use of radiation. 
It consists of a series of narrow-wavelength bands 
in the infrared, modulation to be accomplished by 
simultaneous shifting of the wavelengths of all these 
bands, first to longer wavelengths, then to shorter, 
then to longer, etc., by means of a vibrating-mirror 
arrangement. The security of the system lies in the 
fact that the intensity of the beam remains sensibly 
constant and no photosensitive receiver can receive 
the signal unless equipped with a suitable device 
to convert the spectral modulation into amplitude 
modulation. 

Transmitter. The method of modulation is shown 
in Figure 34. The light from a continuous-spectrum 
source, such as a tungsten lamp, is spread by a 
dispersing system into a spectrum. This falls on a 
grid of alternate opaque and transparent lines, 
which permit selected wavelength bands to pass. 
These are reunited into an achromatic beam by a 
second identical dispersing system. A vibrating 
mirror between the first dispersing element and the 
grid is used for audio modulation of the wave¬ 
lengths passed. 

A number of transparent grid lines, probably at 
least six, must be used with this system. If one or 
only a few wavelength bands are transmitted, their 
motion in the spectrum will be converted to ampli¬ 
tude modulation in any receiver if they happen to 
fall at a point where the slope of the detector re¬ 
sponse curve or the slope of a receiver filter spectral 


transmission curve is very steep. The magnitude 
of this effect decreases with increase in the number 
of wavelength bapds used. To avoid such an effect 
the German spectral-modulation blinker systems 
(Section 4.2.1) employed modulation of a narrow 
dark band in the spectrum, not modulation of nar¬ 
row bright bands. 

No filter over the source would be needed with 
the present spectral modulation system since the 
grid can be used to cut off the spectrum at any 
desired wavelength. 



For highest intensity the width d of the source 
(or entrance slit) should be approximately equal to 
the width of the grid lines. The smallest possible 
beam width is then d/f, where / is the common 
focal length of the three lenses. In the system 
shown the emergent beam varies in direction 
slightly during modulation by an angle somewhat 
smaller than d/f. The beam width used should be, 
therefore, several times wider than d/f , so that this 
oscillation will not interfere with reception. The 










162 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


width may be obtained by defocusing the last lens 
or by use of a “beam spreader” device. 

Receiver. Reception of the signal is obtained by 
use of an identical dispersing system and identical 
grid over the photodetector, so that, as the wave¬ 
length bands of the incoming light vary, they move 
across the grid and the light transmitted through 
the grid to the photodetector is amplitude-modu¬ 
lated. 

Security. If the grid over the photodetector has 
the wrong spacing or line width, a great loss in 
range results so that the system has message secu¬ 
rity against interception even by an enemy receiver 
of the same kind unless the grid of the latter system 
is of exactly the right design. Without difficulty the 
source and receiver grid spacings on all systems 
could be occasionally changed to make interception 
less easy. 

Modifications. Multichannel communication is 
evidently possible, each channel having a different 
grid spacing. Several interchangeable grids could 
be placed on source and receiver for channel selec¬ 
tion. The number of noninterfering channels on a 
feasible system is not large, perhaps less than half 
a dozen. 

Several variants are possible in the source modu¬ 
lation system. In one, the grid might be made to 
move. It seems unlikely that this could be made 
efficient because the amplitudes of mechanical trans¬ 
lation which could be obtained easily at voice fre¬ 
quencies are of the order of a few thousandths of an 
inch while the line width d required for ease of 
adjustment would be several times this, making 
the modulation ratio small. Another variant would 
be to use the vibrating-mirror system but dispense 
with the second dispersing system, reflecting the 
desired wavelength bands back through the first 
system by a reflecting grid. The returning beam 
could then be reflected in the desired direction by 
a semitransparent mirror, but at some loss in effi¬ 
ciency. Both these variants have the advantage that 
the emergent beam would not oscillate in direction 
with the modulation, and therefore it could be 
reduced to the minimum width d/f. 

Evaluation. The system can be made from com¬ 
mercially available components. It is only slightly 
more complex than ordinary vibrating-mirror sys¬ 
tems and may be almost as efficient. The proposal 
deserves further study as a supplement to such 
systems. 


48 POSSIBLE INTERMEDIATE 

INFRARED [HR] COMMUNICATION 
SYSTEMS 

General Possibilities 

Scope of Work to Date. Other types of infrared 
voice communication systems can now be devised 
which use radiation in an entirely different wave¬ 
length region to carry the communication. This 
region is the intermediate infrared between 1.4 and 
6.0 p. It lies beyond the 0.8 to 1.4 p region involved 
in the near infrared systems previously discussed. 
Such IIR systems have been made possible by the 
invention of the lead sulfide photoconductive cell, 
the first sensitive detector of radiation beyond 1.4 p 
to have a response time short enough to pick up 
audio-frequency signals. 

Considerable space will be devoted at this point 
to the possibilities of the IIR. This will be done, 
first, because it is an important region, the exploit¬ 
ing of which will almost double the military useful¬ 
ness of the infrared for communication, since every 
NIR system has in principle an IIR counterpart, 
to say nothing of the value of the region for pur¬ 
poses of heat detection (Chapter 9); second, at 
this time, IIR systems are just entering the labora¬ 
tory and development stage, and many mistakes 
and false hopes may be avoided by a clear under¬ 
standing of the relative advantages and limitations 
of, and the unique conditions imposed on, work in 
the IIR. 

The discussion is based on survey measurements 
and preliminary calculations made under Contracts 
OEMsr-60 (Harvard University), OEMsr-990 and 
OEMsr-235 (Northwestern University). 19 ’ 32 - 33 The 
object of this work was to examine the feasibility 
of IIR systems and to investigate suitable sources, 
methods of modulation, filters, atmospheric trans¬ 
mission, and expected ranges. The termination of 
the war interrupted this work before an IIR voice 
communication trial system was actually built, so 
the conclusions here must be regarded as very 
tentative. Further work on the same lines and con¬ 
struction of trial systems for military use is highly 
recommended. 

Advantages of the Region. By using radiation in 
the IIR, with the visible and NIR radiation elim¬ 
inated by filters, a communication system can be 
made which has, at present, excellent system secu¬ 
rity, i.e., which is secure against enemy detection 





POSSIBLE INTERMEDIATE INFRARED [IIR] COMMUNICATION SYSTEMS 


163 


by any present infrared electron telescopes or by 
phototube or TF-cell receivers. The only detectors 
for such a system known to the enemy would be 
the German PbS cells or other longer wavelength 
cells as yet incompletely developed. This compara¬ 
tive security may not last long, but in any case a 
new and independent channel of communication 
has been opened up and should be utilized. 

Disadvantages of the Region. A possible disad¬ 
vantage of IIR communication, the importance of 
which cannot yet be assessed, is the sensitivity of 
the PbS cell to low-temperature heat sources (such 
as engine exhausts in its field of view), as a result 
of its long wavelength response. The noise produced 
in a receiver system by such sources may prove 
troublesome. 

Enemy Use. The German Lichtsprecher systems 
(Section 4.3.1) have for several years used PbS-cell 
receivers with a tungsten source. The effective com¬ 
munication wavelengths lie in both the NIR and 
IIR. The systems operate with beams of about 
*4 degree and are probably sufficiently secure with¬ 
out elimination of the NIR. If necessary, the NIR 
could be removed by a suitable filter with little loss 
in range. 

Lead Sulfide Cells as IIR Detectors 

Properties. The lead sulfide photoconductive cell, 
developed in America under Contract OEMsr-235, 
is described in Chapter 3. Its use as an NIR de¬ 
tector is discussed under “Daylight Operation,” 
Section 4.1.3, and as a detector of heat sources in 
Chapter 9. The properties of the cell already men¬ 
tioned as important for voice communication work 
are (1) insensitivity to background light, (2) gain 
in sensitivity at low temperatures, and (3) flat fre¬ 
quency response. The additional property impor¬ 
tant for the IIR is (4) its response to wavelengths 
extending from the visible to beyond 3 p. Two 
fairly typical response curves are shown in Figure 
35 A. 

Other types of cells sensitive to even longer IIR 
wavelengths have been produced by the same con¬ 
tract but not in quantity (Chapter 3). Their prop¬ 
erties have not yet been examined as carefully as 
those of PbS. They will therefore not be considered 
here, but they deserve much further study as IIR 
detectors. 

Sensitivity. Unfortunately, the most sensitive 
PbS cells yet produced have S/N ratios averaging 


about 25 db lower than TF cells of the same size 
for the same modulated input energy of wave¬ 
lengths in the sensitive range. This is not so serious 
as it would seem, if a continuous-spectrum source 
is used, because a much larger fraction of the radi¬ 



100 

80 

60 

40 

20 

0 


c 

ry 


S7T 

V7T 

A 

A “ 

7 - A 





/ \ 

J 

\ 

/ATMOSPHERIC 

/TRANSMISSION 





rp 



r c.t uivi rr i 

/ h 2 o vapor 

1 

\ 


i j 

nj 

m 

w 32 

/ 

* \ 

J IN PATH 



i ! 

-1- 

r 

V 

_ 




12 3 4 


MICRONS 

Figure 35. Response of cells and transmission in the 
IIR. 


ated energy is emitted in the wide wavelength 
response band of the PbS cell than in the narrow 
one of the TF cell. For a tungsten lamp source at 
2848 K, covered by 2.0 millimeters of Corning 
2540 filter, the PbS S/N ratio is only 10 db less 
than the TF S/N ratio (20 db if no filter is used). 
If the lamp temperature were reduced to about 
1500 K, keeping the same filter, the PbS value 
should become some 15 db better than the TF 
value. These figures are for a 90-cycle signal. Pre¬ 
sumably the relations remain approximately the 
same at the speech frequencies near 1,500 cycles 
which are important for intelligibility since the S/N 
ratios are independent of frequency (see Chap¬ 
ter 3). 

The PbS cells are still very new, and their sensi¬ 
tivity may be greatly increased with further devel¬ 
opment. 



































































164 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


Area. Most of the PbS cells produced under Con¬ 
tract OEMsr-235 have had sensitive areas ^4 inch 
square or smaller, suitable for use in narrow-beam 
systems; but one or two cells have been made fairly 
successfully with areas of more than 2 square 
inches. 

Atmospheric Transmission in the HR Region 

The absorption of water vapor in the atmos¬ 
phere is the primary determinant of the wavelength 
bands in which IIR communication can be carried 
on. The effects of source distribution, filters, and 
cell-response cutoffs are subordinate. 

Methods of Measurement. The transmission of 
solar radiation from 0.8 to 7.0 \i through the water 
vapor in the atmosphere was measured under Con¬ 
tract OEMsr-990 with a Perkin-Elmer infrared 
spectrometer. 18 Harvard Contract OEMsr-60 3233 
set up what was essentially a reflecting telescope 
with a recording bolometer receiver, the telescope 
being covered by a thin glass prism, for studying 
the transmitted spectrum from a tungsten lamp 
source 5,000 yards away across Broad Sound, in the 
vicinity of Boston. The wavelength range accessible 
with this apparatus was from 0.4 to 2.7 p. The 
transmission of solar radiation as a function of 
solar altitude (air mass) was also studied care¬ 
fully. 

The quantity of water vapor in the path, reduced 
to centimeters of precipitable water [cm ppt H 2 0], 
was estimated in the first contract from meteoro¬ 
logical data and was determined in the second one 
from wet and dry bulb measurements with a sling 
psychrometer. A quantity of 1.0 cm ppt H 2 0 is 
approximately the amount of water vapor traversed 
in a 1,000-yard path, at 20 C and 50 per cent rela¬ 
tive humidity, which we may tentatively define as 
“average clear weather” for IIR communication 
purposes. 

Water Vapor Transmission. The results of the 
two studies are in substantial agreement with each 
other and with measurements in the literature. 34 ’ 35 ’ 36 
Lambert’s law that the same fraction of the radia¬ 
tion is absorbed in each successive element of dis¬ 
tance holds very well for the transmission through 
haze in the NIR but is not even approximately 
obeyed in the IIR. The rule in the IIR is that the 
bulk of the absorption in average clear weather 
takes place in the first half mile, and that the total 
change thereafter amounts to only a very few per 


cent for paths up to several miles. This is shown 
by Table 4 which gives integrated transmission 
values determined from the spectrograms recorded 
under Contract OEMsr-60. 


Table 4. Fraction of incident energy transmitted by 
water vapor in near and intermediate infrared regions. 


cm ppt HaO 

I 

II 

Region * 

III 

IV 

V 

1.1 

0.74 

0.81 

0.71 

0.64 

0.57 

3.6 

0.64 

0.64 

0.51 

0.52 

0.50 

7.5 

0.51 

0.56 

0.44 

0.51 

0.48 


* See Figure 35. 


The regions indicated are those marked in Figure 
35 (bottom), which shows the transmission of solar 
radiation as a function of wavelength with about 
2.7 cm ppt H 2 0 in the path, as estimated under 
Contract OEMsr-990. The integrated transmissions 
over these regions on Figure 35 agree roughly with 
those in Table 4, the latter values being much more 
accurate because more careful determinations of the 
absolute incident and emergent energy distributions 
were made. Regions I, II, and III are in the NIR; 
Regions IV and V in the IIR. For the last two 
regions, to a good approximation, we may take the 
loss of signal by atmospheric absorption and scat¬ 
tering to be about 6 db in the first half mile and 
zero thereafter in average clear weather. 

Reason for Departure from Lambert 1 s Law. Lam¬ 
bert’s law is violated in the IIR because the absorp¬ 
tion is not produced by a continuous spectrum but 
by a large number of very sharp and intense lines 
the natural widths of which are much narrower than 
the distances between them. The centers of the lines 
are completely absorbed by the water vapor in 
paths of just a few feet, and the radiation passed 
comes through many narrow gaps between adjacent 
lines. Theoretically, for lines of equal intensity and 
spacing, the fraction absorbed is the ratio of the 
effective line width to the gap width. This fraction 
grows at the rate at which the relative line width 
grows, that is, approximately, only as the square 
root of the path length for transmission values be¬ 
tween about 0.1 and 0.9 p. 34 * 35 Where the lines are of 
equal spacing the absorption increases more slowly. 
Actually, in the IIR, with the relevant transmis¬ 
sions integrated over both strong absorption bands 







POSSIBLE INTERMEDIATE INFRARED [IIR] COMMUNICATION SYSTEMS 


165 


and almost transparent regions, after the first great 
decrease the transmission changes very slowly in¬ 
deed. 

Haze Losses. Those working under Contract 
OEMsr-60 found the losses from scattering by 
haze to be negligible at 2.5 p. At shorter wave¬ 
lengths the losses increase strongly with decreasing 
wavelength, becoming as great as 20 db at 0.68 p 
for the 5,000-yard path length in weather which 
was still clear enough for making spectrograms. 

Transmission through Chemical Smokes. These 
results are confirmed by tests with smokes. The 
Bureau of Ordnance cooperated with those working 
under Contract OEMsr-60 in testing reception 
through large quantities of various kinds of chem¬ 
ical fogs and smokes in the path over Broad Sound. 
Smoke which would have made NIR communication 
completely impossible would have had little effect 
on IIR communication. For example, a plume of HC 
(hexacliloroethane, zinc and zinc oxide) smoke 
from a standard smoke pot, which was completely 
opaque visually to a 1,000-watt searchlight, attenu¬ 
ated the beam on the average more than 20 db at 
0.85 p, but only 4 db at 2.1 p. 33 The results with 
other smokes were similar; details may be found in 
the original report. 

Comparison of Atmospheric Transmission in the 
Near Infrared and Intermediate Infrared. A con¬ 
tinuous-spectrum IIR system evidently has a con¬ 
siderable advantage in atmospheric transmission 
over an NIR system, for which the haze transmis¬ 
sion losses increase regularly (Lambert’s law) at 
about 4.5 db per mile in average clear weather. An 
IIR system could start with an initial handicap, 
say, of 20 db in overall performance compared to an 
NIR system. The IIR system would lose only the 
additional 6 db in the first half mile, while the 
losses of the NIR would steadily increase. The per¬ 
formance of the systems would become equal at 
ranges of about six miles in average weather. At 
longer ranges the IIR system would excel because it 
would not be nearly so much affected by weather 
as the NIR system. 

It is worthy of note that the water vapor absorp¬ 
tion losses in the IIR actually may be less serious 
even than those in the FIR (8 to 13 p), which is a 
region usually considered quite transparent. This is 
easily understood when it is realized that the gaps 
between the water vapor bands in the IIR, if ex¬ 
pressed in the more fundamental units of frequency 


rather than of wavelength, afe actually wider than 
the gap in the FIR between the 6 and 25 p bands. 
Also the intensity of the IIR bands is lower than 
that of the FIR bands. The result, for which there 
seems to be some experimental evidence, is that the 
atmosphere should be more transparent in the IIR 
than in the FIR. 

Use of Glass or Plastic Optical Elements 

Glasses and plastics always have strong absorp¬ 
tion bands near 3 p. The absorption bands of glass are 
exhibited in the curves of Figure 35 for thin Corn¬ 
ing 430 and Nonex samples. It was thought at first 
that these bands might be serious enough to require 
the elimination of all such material from the optical 
path. But since in just a short path the 2.7-p water 
vapor band absorbs all the wavelengths in this 
vicinity anyway, it seems that moderate thicknesses 
of glass or plastics can be tolerated. [This is true 
only for the IIR communication problem; for heat 
detection with PbS cells it may be essential to elim¬ 
inate all glass and plastics from the path, so as not 
to diminish in any way the small but important 
longest-wave tail response to radiation transmitted 
through Region VI (see Chapter 9).] For communi¬ 
cation, probably as much as two centimeters of 
good optical glass can be used in the path without 
any serious effect on range. As filters and cover 
glasses alone may add up to about this thickness, 
probably mirrors rather than lenses should be used 
in the IIR optical systems for collimation and 
focusing. A design like that of type W is better in 
this respect than any of the other vibrating-mirror 
designs. 

Shift of Long Wavelength Cutoff in Glass Cells. 
A related problem is that of the effect produced by 
the difference of the response curves for melted PbS 
layers in air and thin evaporated layers inside a 
Nonex cell. The latter type, represented by NU 2126 
in Figure 35, can be made more reproducibly, but 
they have a shorter wavelength cutoff as shown in 
Figure 35. However, the water vapor absorbs most 
of the wavelengths in the neighborhood of this 
long-wave cutoff and the shorter wavelength of the 
cutoff produces no appreciable loss in communica¬ 
tion range. The difference of the response curves is 
due not only to the absorption of the Nonex but 
also to the failure of the thin evaporated PbS layer 
to absorb the longer wavelengths, as shown in the 
PbS transmission curve at the top of Figure 35. 



166 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


Filters 

Filters for the HR must transmit very little below 
1.4 p if they are to give security against enemy 
detection by phototubes, TF cells, and image tubes, 
and very well in Regions IV and V if they are to 
give maximum communication range. 

Types. Various Corning glass filters in blue and 
blue-green of the proper thicknesses promise to give 
adequate differentials in transmission between the 
NIR and the HR. Plastic filters, such as the XRF-1 
types developed under Contract OEMsr-1085 for 
this problem may also be satisfactory (see Chap¬ 
ter 2). 

Transmission Curves. The infrared transmission 
curves of the suitable glasses and plastics tested are 
almost identical. The curve for Corning 430, 2 milli¬ 
meters thick, is shown in Figure 35B. 

The significant differences in the curves of vari¬ 
ous samples are those which occur in the important 
region of very low transmission near 1.2 p and 
would not show in this figure. Because of the small 
transmission at this wavelength, it is essential, in 
measuring transmission curves for comparison of 
HR filters, to use a double monochromator in this 
neighborhood. Scattered background light will cause 
serious errors with singly dispersing instruments 
like the commercial infrared spectrometers. 

The Corning glasses have some blue and green 
visual transmission which can be eliminated by the 
addition of Corning 2540 or 2550. The curve for 
2540 in the NIR is also shown in Figure 35. These 
glasses have negligible absorption in Regions IV and 
V. They may be made as thick as necessary to limit 
the NVR to the desired values, without having any 
great effect on the HR range. 

Holotransmissions for PbS Cells and TF Cells. 
The best filter tested under Contract OEMsr-990 
consisted of 2-millimeter Corning 430 plus 2-milli¬ 
meter 2540. The holotransmissions of this combina¬ 
tion and of 2540 alone are given in Table 5A for TF 
and PbS cells. The values were measured on the 
photocell test set (Chapter 3); the source is an 
unfiltered tungsten lamp at 2848 degrees K, with the 
light chopped by a sector disk at 90 cycles. 

The S/N ratio of a good PbS cell at present is 
about 20 db less for this source (unfiltered) than 
for a TF cell of the same size. Taking the TF cell 
S/N as a reference level, the S/N ratios of the two 
cells with filters are as shown in Table 5B. 


Range computations can be made from these data 
only after a source is chosen for the IIR work. 


Table 5. Comparison of NIR and IIR sources, filters, 
and cells. 


Cell 

No filter 

2540 alone 

430 plus 2540 


A. 

ehT values 



(given as decibel loss) 


TF 

0 db 

—15 db 

—75 db 

PbS 

0 db 

— 5 db 

—20 db 


B. 

S/N ratio 


(relative to value for TF cell with white light, 2848°T C ) 

TF 

0 db 

—15 db 

—75 db 

PbS 

—20 db 

—25 db 

-^0 db 


C. S/N ratios for tungsten source at 1500 K 

(relative to value for TF cell with white light at 2848° T c 


at same source power) 


TF 

—15 db 

—30 db 

—90 db 

PbS 

—10 db 

—15 db 

—30 db 

D. 

Estimated reception ranges with 1500 K tungsten source 


(assuming PbS range of 20 miles with 

no filter) 


Vacuum range 


TF 

15 miles 

6 miles 

400 yd 

PbS 

20 miles 

15 miles 

6 miles 


Average clear weather range 


TF 

4.5 miles 

3 miles 

400 yd 

PbS 

14 miles 

10 miles 

4 miles 


Sources 

From the transmission curve of Corning 430 plus 
the water vapor in a long path (Figure 35, shaded 
area), it is seen that the wavelengths effective in 
carrying the communication in a secure PbS system 
are near 1.7 and 2.2 p. An efficient IIR source must 
radiate a large fraction of its energy in these re¬ 
gions and must be suitable for modulation. 

Gas Discharge. Xenon and krypton (or other 
rare gas) sources like those used in the Touvet 
system and the V-M system may be capable of 
emitting electrically modulated radiation in these 
regions under certain excitation conditions (see 
“Source/’ Section 4.5.3). Very little is known about 
this possibility. 

Mechanical Modulation. A source which would 
certainly work is a tungsten lamp with its radiation 
mechanically modulated by a vibrating mirror as in 
the type G and type W systems. 

For greatest efficiency with such a source, the 
lamp temperature should be adjusted to about 1500 
K, so that the maximum of the radiation curve will 
be near 2 p. An input power of, say, 100 watts at 










POSSIBLE INTERMEDIATE INFRARED [IIR] COMMUNICATION SYSTEMS 


167 


this temperature would require a source several 
millimeters in diameter. This is large but not pro¬ 
hibitive for a vibrating-mirror system. A feasible 
beam width in this case would be 1 or 2 degrees or 
larger. With increase of temperature from this 
value, the beam solid angle decreases faster than 
the efficiency, so that the effective IIR holocandle- 
power and the range are both greater at the higher 
temperature for the same source power. This is 
probably the reason for the use of tungsten lamps 
near 3000 K in the German Lichtsprecher, even with 
PbS cell receivers. 

A rough calculation shows that the output from 
a 1500 K source in Regions IV and V would be 
about 10 db more than from a source of the same 
power at a temperature near 3000 K, while the 
output near 1.0 p would be about 15 db less. These 
may be taken to be roughly the differences pro¬ 
duced in PbS and TF cell responses, respectively, 
if a 1,500-degree source were substituted for the 
source in the photocell test set. 

Range and Security 

A PbS-cell tungsten-lamp IIR system with suit¬ 
able filters would be simpler to make than an NIR 
system. The small PbS cells now made are well 
adapted for use in a narrow beam system which has 
its own kind of security. One or more of the devices 
considered for increasing the security of the NIR 
system could also be added later to an IIR system 
if needed. 

The range and security of the PbS-cell tungsten- 
lamp system can be calculated with the help of 
certain assumptions. 

Detection by TF-Cell Receiver. The range of 
detection of the source with a TF-cell receiver can 
be estimated by comparing the range of TF-cell 
reception with that of PbS-cell communication re¬ 
ception. Applying the above corrections for source 
temperature to the relative S/N values given in 
Table 5B, we then have for a source at 1500 K the 
values given in Table 5C. Assume that an IIR sys¬ 
tem with PbS cell and 1,500-degree tungsten source 
can be constructed which has a vacuum range of 
20 miles with no filter. Then, on the assumption that 
PbS and TF cells of the same size are interchange¬ 
able in the receiver, the communication ranges can 
be computed from Table 5C and are given in Table 
5D. In average clear weather, with the 430 plus 2540 
filter combination, the IIR communication range is 


4 miles, the NIR range 400 yards. The very approxi¬ 
mate nature of these estimates must be emphasized. 

To give the ran^e at which it could be detected 
that a modulated infrared source was operating, the 
range of 400 yards, which refers to TF cell for voice 
reception, should probably be doubled. Enemy use 
of very large receiver areas of two or three feet in 
diameter would give ranges several times larger 
still, perhaps up to a mile or more. This is of 
course the maximum NIR detection range in the 
center of a beam 1 or 2 degrees wide. 

A more dense filter than the 430 used in this 
calculation would give lower TF cell ranges, with 
little effect on PbS ranges. Of course, the thinnest 
filter that could be permitted should be used in 
order to keep the PbS ranges as great as possible. 

Detection by Phototube Receivers and Electron 
Telescopes . Phototube receivers and electron tele¬ 
scopes give detection ranges comparable with those 
of TF cell receivers for NIR sources. An IIR filter, 
however, affects their response to an IIR source 
much more than it affects the response of a TF cell, 
because of the longer wavelength threshold of the 
latter. The differential factor is estimated to be of 
the order of 10, when Cs-Ag-0 Si-type photoemis- 
sive surfaces are considered. With such a factor, the 
detection ranges with these devices should be only 
about one-third of the ranges obtained with TF-cell 
receivers. 

Visual Range. The visual range of an IIR sys¬ 
tem may be reduced to as small a value as desired 
by use of sufficient thicknesses of 2540 filter, with 
little effect on the IIR range. 

Intermediate Infrared Communication Range. 
The vacuum range assumed to be feasible for pur¬ 
poses of computation, for the PbS system in Table 
5D, is not excessive as can be seen by changing this 
system over to an NIR system and comparing it 
with known NIR systems. If the tungsten source 
used in computing the table were replaced by one 
of the same power at 2848 K with the same beam 
width, the TF-cell vacuum range of the imagined 
system with the 2540 filter would become about 14 
miles. Such a range is not much greater than that 
expected for type W with a 3-degree beam, and it is 
considerably less than that expected for type G with 
a 100-watt source, a Corning 2540 filter, and a beam 
width between 1 degree and 4 degrees. The latter, 
if converted to IIR, should therefore give a larger 
range than the system imagined in Table 5D. 


n_ H '-UEtg 





168 


NEAR INFRARED VOICE-CODE COMMUNICATION SYSTEMS 


Because of the low attenuation, an increase of 
source power (or beam intensity) in the HR is 
much more effective in increasing the range than in 
the NIR. A factor of two in source power in the 
NIR produces a change of about 15 per cent in the 
range of an NIR system whose ACW range is about 
5 miles; in the IIR, the change produced would be 
about 40 per cent. 

Possible Interference. The possibility of an IIR 
voice system encountering serious interference from 
low-temperature engine exhausts in the field of view 
has already been mentioned. Only operational tests 
can determine the importance of such interference. 
Filtering out the strong low-frequency components 
of such noises in the receiver amplifier might give 
adequate relief. 


49 SUMMARY OF RECOMMENDATIONS 

If development of infrared communication sys¬ 
tems is to be continued, some particular and general 
recommendations can be made. The particular ones 
concern continuation of the work in progress at the 
termination of World War II: 


System 


Recommendation 


Type W Further tests 

Type E None; system in manufacture 

P-G Further tests 

P-P Completion and tests 

V-M None; no features not duplicated in 

other systems 

Touvet Study of source 

Elimination of a-f modulation 
Improvement of receiver angles 
Photoelastic shutter No further development; tests of 
Navy pilot models 

Spectral modulation Construction and study 


Further general recommendations can be made 
on fields of interest and promise which deserve 
study by the Armed Services for possible future 
military applications. It is assumed that the Serv¬ 
ices will keep abreast of source, filter, photocell, and 


circuit developments. It is to be hoped that careful 
study will be given to each of these various systems 
by other branches of the Services than those for 
which they were developed. The following prob¬ 
lems should be undertaken: 

1. Application of intelligibility-band-pass stud¬ 
ies 41 to the above systems. 

2. Increased security: modification of above 
systems for greater security and construction of 
new ones from this point of view. 

3. Adaptation of above systems to perform rec¬ 
ognition function (see also Sections 5.2 and 5.6). 

4. Direct comparison of weight, stability, and 
efficiency, etc. of electrically modulated and vibrat- 
ing-mirror systems designed for the same range, 
angles, and security. 

5. Temperature and altitude effects, and engine 
exhaust difficulties, with various detector cells. 

6. Daylight communication: PbS cells and 
others. 

7. IIR systems: PbS cells and others. 

8. NIR (beacon) and IIR (heat radiation) 
tracking devices: with TF and PbS cells and others. 

A great number of these infrared communication 
devices have now reached, or almost reached, the 
quantity production stage. But as yet little atten¬ 
tion seems to have been paid to the fact that many 
of them ‘will interfere greatly with each other if 
they are ever used close together in the field. This 
situation becomes more serious with the addition of 
the systems described in Chapter 5. Before further 
production or development continues, the various 
branches of the Armed Services should immediately 
take steps to channel all their infrared efforts 
through a single agency which could allocate fre¬ 
quencies, wavelengths, and signaling methods to the 
various devices; otherwise, mutual interference, 
noise, backscatter, and cross talk may prevent any 
of these equipments from attaining their ultimate 
performance in the field. Further remarks on this 
subject will be found in Section 5.6. 




SUMMARY 


169 


a 

a 

do 

do 

A 

At 

a 

B 

3 

d 

A/ 

e r 

e t 

ei 


E 

i 

F 

0 

h 

1 

h 


Or 

Ot 

Qr 

Q o 

V 


SYMBOLS IN EQUATIONS 

(Photometric and holophotometric symbols not distinguished) 


For Section 4.1.3 

total photoresponsive surface area of detector 
cell 

maximum projected photoresponsive area of 
bare cell 

total effective luminous surface area of source 
maximum projected area of bare source 
receiver entrance pupil 
exit pupil of transmitter 
hpi transmitter beam width 
surface brightness of source 
hpr width of receiver directivity pattern 
distance between transmitter and receiver axes 
(in a transceiver head) A, A’, 

receiver bandwidth 
receiver efficiency (optical) 
transmitter efficiency (optical) 
product of transmission and reflection coeffi¬ 
cients in transmitter system 
illumination (flux/unit area) on a surface in the 
transmitter beam 

focal length of transmitter optical system 
threshold flux on receiver entrance pupil 
total emergent d-c flux from bare source 
ehT of filters in beam 

maximum practical rms variation of beam 
candlepower during modulation 
maximum bare source candlepower 
constants in range equation, including photo¬ 
cell noise, geometry factors and aberration fac¬ 
tors 

receiver optics factor 
transmitter optics factor 

hpr solid angle of receiver directivity pattern 
hpi solid angle of transmitter beam 
object distance: from source or transmitter to 
image-forming, intensity-reducing lens or mir¬ 
ror 


q image distance: from same lens to the image of 
the source or transmitter 
R distance from transmitter to receiver 
Rt operational range (limit) of communication 
system in weather with transmission T 
R v vacuum range of a system (limit) 

T atmospheric transmission (per sea mile) 
z rms variation of intensity relative to mean d-c 
intensity with modulation device removed 


For Section 4.6.2 
B, B' emergent beams 

a phase angle of time variation of supersonic 
wave 

(3 angle by which analyzer must be rotated from 
crossed position to produce given transmitter 
intensity with no modulation 
Aji difference in refractive indices of birefringent 
<p medium 

(p'n optical phase shift (introduced by birefringent 
medium) between components of light parallel 
and perpendicular to optic axes 
<pm maximum of d at an antinode at a given 
instant 

i|> m maximum of reached during an r-f cycle 
/ instantaneous intensity transmitted through 
crossed Polaroids 

Io intensity transmitted through parallel Polaroids 
(no modulation) 

It average of I over one r-f cycle 
Jo Bessel function of order zero 
p, q Neumann’s photoelastic constants 
t time; thickness of glass block 
x period of supersonic wave 
0 phase angle of supersonic wave as function of 
position on block 




Chapter 5 


NEAR INFRARED RECOGNITION AND CODE 
COMMUNICATION SYSTEMS 

By John R. Platt a 


5.1 INTRODUCTION 

I n the present chapter is collected an assortment 
of near infrared [NIR] systems which are sim¬ 
ilar in basic principle to the voice and code com¬ 
munication systems of Chapter 4, but which have 
two fundamental differences. The systems of this 
chapter all transmit and receive no voice but only 
low-frequency code signals, from 90 to 600 cycles, 
and they each perform some function other than 
that of communication. 

The type D and D-2 systems (Section 5.2) began 
as identification units, that is, the first models were 
designed with 360-degree transmitters to broadcast 
national or specific identification call letters over 
and over, almost continuously in all directions. The 
function of receivers in such systems is to determine 
whether an object or target in the field of view has 
or has not such identification. Although the em¬ 
phasis in this type D development was later concen¬ 
trated on rapid code communication, identification 
was retained as a secondary function. 

The plane-to-plane recognition [PR] system 
(Section 5.3) performs only the identification func¬ 
tion. 

The retrodirective target locator [RTL] (Section 
5.4) uses infrared radiation to locate certain special 
mirrors in the field of view; it was designed orig¬ 
inally to locate life rafts at sea. 

In the Japanese infrared detection [JAPIR] sys¬ 
tem (Section 5.5), the transmitter is omitted and 
the function of the system is reduced to the detec¬ 
tion of enemy infrared sources. 

The general remarks on types of systems given 
in Section 4.1.2, on components and ranges in Sec¬ 
tion 4.1.3, and on the state of the art at the begin¬ 
ning of the National Defense Research Committee 
[NDRC] program as given in Section 4.2 may also 
be applied to the systems to be discussed in this 
chapter. The four equipments to be described here 

a Northwestern University, Evanston, Ill. Now at Univer¬ 
sity of Chicago, Chicago, Ill. 


are rather closely related and are all low-audio¬ 
frequency intensity-modulation systems, so that 
further breakdown into categories like those of Sec¬ 
tion 4.1.2 is unnecessary (see Table 1 of Chapter 4). 
Two of the systems employ electrical modulation 
of a tungsten filament source; the others modulate 
the beam with a mechanical rotating sector. 

The other methods of modulation and methods of 
obtaining greater security are described in Section 
4.1.2, and include polarization or wavelength modu¬ 
lation, which could also be applied to the problem 
of recognition and identification. Most of these 
other methods entail an increase in weight and com¬ 
plexity; but one method leading to greater security 
—the use of the intermediate infrared [HR]—could 
be introduced immediately, by a simple interchange 
of filters and photodetector cells. 

As many of the voice systems described in Chap¬ 
ter 4 had provision for code communication, it might 
seem at first glance that the code systems to be 
described in this chapter are an unnecessary dupli¬ 
cation. This is partly true, so far as code com¬ 
munication alone is concerned, but none of the sys¬ 
tems of Chapter 4, except possibly the P-P system 
(Section 4.4.4), have the 360-degree transmitters 
required to perform the identification function which 
is performed by the type D, D-2, and PR systems 
to be described. 

The fact that most of these all-around identifica¬ 
tion systems are designed only for code and not for 
voice is a result of the general economics of weight, 
power, and beam angle in signaling systems. As dis¬ 
cussed in Section 4.1.3, a wide-angle system is a 
short-range system. To get militarily useful ranges, 
considerable source power must be used, 1,000 to 
1,300 watts in the type D development, for exam¬ 
ple. This large essential drain of power puts a 
premium on simplicity of modulation methods and 
considerably favors simple electrical or mechanical 
coding, as compared with voice modulation which 
involves comparatively large and heavy power am¬ 
plifiers. To get around this difficulty, it has been 


170 



INTRODUCTION 


171 


proposed to use 10- to 40-degree directed-beam low- 
power transmitters which could sweep the horizon 
continuously with a code signal for identification 
and could stop for challenging and voice communi¬ 
cation. This possibility has apparently not been ex¬ 
plored very far. It, together with some other possi¬ 
bilities for amalgamating the identification and 
voice systems in order to make a great saving of 
weight and power, needs study from the point of 
view of all the military requirements, including the 
total shipboard or aircraft infrared installation. 
This question will be discussed further in Section 
5.2 under “Evaluation” and “Recommendations,” 
and in Section 5.6. 

5,11 Military Applications 

The first important military application of the 
systems described in this chapter is the equipping 
of ships and planes with all-around infrared beacons 
and directed receivers so that the craft are imme¬ 
diately identified as friendly by any other similarly 
equipped ship or plane within range. The second 
important application is the use of these same 
beacons and receivers for challenge and all-around 
(code) communication after identification has been 
established. The same principle can, of course, be 
applied to tanks, infantry units, and front-line 
trench positions. The advantages of the infrared 
in security for military situations requiring radio 
and radar silence have already been described in 
Section 4.1.2. 

Examples of both of these applications are type 
D and its later and more useful successor, type D-2 
(Section 5.2). The complete production model of a 
D-2 system, for ship-to-ship use, weighs about 1,200 
pounds and has eight 170-watt transmitter beacons 
for 90-cycle code, giving 360-degree horizontal 
coverage with a 50-degree vertical half-peak-inten¬ 
sity [hpi] width. It has two automatically scanning 
receiving heads with half-peak-response [hpr] 
angles of about 12 degrees. The average clear 
weather [ACW] range (see Section 4.1.3) is not 
known exactly, but is estimated to be about 6.5 
sea miles. 

An example of the identification and recognition 
application without code communication is given by 
the PR system. One complete aircraft installation 
weighs about 25 pounds and has four 6-watt tung¬ 
sten lamps, coded at 90 cycles, one mounted on the 


top and bottom of each wing tip for all-around 
(spherical) coverage, and a receiver with a 15-degree 
hpr angle. Obtainable ACW ranges are expected to 
be greater than 2,500 yards for every direction of 
the receiver from the source lamps. The receiver 
can probably be mounted on gimbals and elec¬ 
trically aligned with the plane’s guns so as to give a 
warning signal if they are trained on a friendly 
target. The ACW ranges in either direction between 
the type D system on a ship and the PR system on 
a plane are expected to be about 12,000 yards. 

The all-around source required for identification 
from all directions gives these systems a somewhat 
lower security against enemy detection and demands 
much more power than is needed for a narrow- 
beam system of the same range. Thus all-around 
type D requires about 1,300 watts of source power 
compared to about 100 watts for the 15-degree type 
E system of about the same range (Section 4.4.2) 
in spite of the much sharper tuning of the type D 
code receiver. 

The RTL system also performs a kind of identifi¬ 
cation function but is suited to a different type of 
military situation. This system has a wide-angle 
transceiver (4 to 15 degrees) at one station only. 
The other station carries one or more precision 
retrodirective triple mirrors which reflect the 90- 
cycle transmitter beam accurately to the receiver. 
The receiver then gives a signal when a mirror is 
in the field of view. These mirrors weigh only *4 
pound apiece and could be attached to life jackets 
for detection during search for survivors at sea, an 
application of civilian as well as military interest. 
They could be fixed to buoys or markers for channel 
location, or carried by landing boats or infantry¬ 
men so as to indicate the progress of an attack to 
a central headquarters station. Ranges up to about 
2 miles were obtained with a working model using 
a 300-watt lamp with an hpi transmitter beam width 
of about 4 degrees. The transceiver station equip¬ 
ment weighs about 115 pounds. In the case of navi¬ 
gation markers and landing craft where some power 
is available to drive a rotating 90-cycle sector 
(coded, if desired) over the triple mirrors, the range 
can be increased to about 4 miles. Replacing the 
receiver in the RTL system with an infrared electron 
telescope for visual detection of the mirrors might 
give comparable ranges and a much more con¬ 
venient apparatus. However, the Navy specifically 
did not want the use of a telescope because of the 




172 


NIR RECOGNITION AND CODE COMMUNICATION SYSTEMS 


required constant attention and consequent opera¬ 
tional fatigue. Such a telescope would necessarily 
add at least one person to a plane crew (the RTL 
was designed for aircraft installation). Where se¬ 
crecy is not essential, an unfiltered beam can be 
used with some increase in range. 

The remaining system to be described here, the 
JAPIR system, has a different function. It was 
designed to be placed on aircraft for detecting 
enemy NIR sources within an hpr angle of about 
6 degrees; it weighs about 25 pounds. Either steady 
or modulated NIR beacons may be detected. Code 
messages from blinker beacons, or from carrier- 
wave sources modulated at frequencies between 60 
and 500 cycles, may be read directly. Voice com¬ 
munication could be monitored if some changes 
were made in the receiver arrangement. The re¬ 
ceiver sensitivity differs by only small factors from 
the sensitivities of other receivers in the other sys¬ 
tems described in Chapter 4 and in this chapter, 
and consequently the maximum range of detection 
of enemy systems will probably be similar to the 
operating ranges of those systems. For high-candle- 
power transmitters like those discussed here (types 
D, D-2, E, and Touvet), the maximum ACW range 
of detection can be computed to be about 9 miles, 
if the JAPIR system were placed in the exact cen¬ 
ter of the beam. In actual operational tests of the 
JAPIR system, a 240-watt NIR-filtered all-around 
lamp mounted on a target plane was only picked 
up by the system at ranges of about 3,000 yards in 
average weather. The JAPIR system will probably 
not detect intermediate infrared transmitters at 
distances of more than a few hundred yards. 

512 Systems not Developed by NDRC 

The code systems to be described here have some¬ 
thing in common with blinker code searchlight or 
beacon systems adapted to the NIR by a suitable 
filter, except that such systems are designed for 
visual observation through an infrared electron 
telescope. Both the Army and Navy have done 
extensive work with such systems. 

Type P 

Code messages may be transmitted by type D or 
type P, a system developed by the Polaroid Corpo¬ 
ration under Navy contracts This system deserves 

b Information supplied by courtesy of Section 660E, 
Bureau of Ships. 


mention here because of the unusual modulation 
method and the elegant presentation of the message 
at the receiving end. 

The type P transmitter is fed by a standard tele¬ 
typewriter, which interrupts a steady line current in 
regular patterns according to the letter struck by 
the operator. The current “breaks” are converted 
into pulses operating a flash lamp NIR source, the 
“makes” into pulses operating a second flash lamp 
source. The two flash lamp transmitters are covered 
by complementary polarizing sheets so that at the 
receiving end the pulses are received alternately by 
two vacuum phototube units covered by comple¬ 
mentary analyzers. The receiver pulses are con¬ 
verted back into “makes” and “breaks” of a steady 
line current operating a receiving teletypewriter, 
which automatically types out the message on a 
tape in the usual way. 

With 3,500-watt lamps, the ACW night range is 
about 6 sea miles with day range about 4 sea miles. 
Speeds up to 60 words per minute are obtainable. 
A similar system of lighter weight is being devel¬ 
oped under Navy contract by Farnsworth Tele¬ 
vision and Radio Corporation. This uses only one 
lamp without polarization, indicating the “makes” 
with one flash and the “breaks” with a double flash. 

52 TYPE D SYSTEM 

Description and Performance 

Course of Development. The type D system, a 
90-cycle code system at present adapted for ship- 
to-ship identification and communication, was de¬ 
veloped by University of Michigan Contract 
NDCrc-185 (Project Control NS-151). This devel¬ 
opment was initiated in October 1942 under the 
Bureau of Aeronautics [BuAer] primarily for ship- 
to -plane recognition and was taken over after Feb¬ 
ruary 1943 by the Bureau of Ships [BuShips], 
chiefly for ship-to-ship code communication. In ex¬ 
plaining the course of this development three stages 
must be distinguished. 

The first stage ran from October 1942 to Feb¬ 
ruary 1943 and was concerned with ship-to-plane 
recognition at distances up to 3 miles. The equip¬ 
ment was tested in the latter month at Norfolk, 
Virginia. A 500-watt mechanically interrupted NIR 
beacon placed on a coastal patrol yacht was iden¬ 
tified with moderate success from a receiver of 14 
degrees hpr width fixed in the nose of a PBY5 





TYPE D SYSTEM 


173 


amphibian plane flying 5 miles or more away. An 
official demonstration was given on the night of 
February 12, 1943, to representatives of BuAer, 
BuShips, and other Army and Navy officers. 1 

There followed the second stage of development 
under Project Control NS-151 requested by Bu¬ 
Ships, in which new laboratory units were designed 
with emphasis on four new objectives: (1) trans¬ 
mission of code messages, (2) ship-to-ship recogni¬ 
tion, (3) automatically scanning receivers, and (4) 
indication of the bearing of a distant source. Subse¬ 
quently commercial Navy contracts were let for the 
quantity manufacture of the type D system based 
upon the design of these units. Samples were tested 
near Solomon’s Island, Maryland, in July 1943. 
Improved laboratory type D units were installed 
on two destroyers, the USS Carmick and the USS 
Corry , and tested near Norfolk, Virginia, on Feb¬ 
ruary 7 and 8, 1944. 2 > 6 The contracts for pilot mod¬ 
els and later for quantity production were made 
with Emerson Radio and Phonograph Company, 
New York City, 4 for type US/D receivers, and with 
Crouse-Hinds Company, Syracuse, New York, 5 for 
type US/D transmitting systems. Contracts were 
also let with GE, Lynn, Massachusetts, and with 
RCA, Indianapolis, Indiana, for beacons and re¬ 
ceivers, respectively. 

The third stage of development followed the Feb¬ 
ruary 1944 tests. Greater emphasis was placed on 
the high-speed communication and break-in func¬ 
tion of the equipment. In this stage were con¬ 
structed laboratory type D-2 units, 3 the production 
of which was taken up by further Navy contracts. 
The transmitters at this stage were completely re¬ 
designed for electrical modulation from a thyratron 
power supply, giving crisp, fast signaling, with off 
periods in which the reply from a distant station 
can break in. Some changes were also introduced in 
the receiver. Satisfactory field tests of the labora¬ 
tory type D-2 system were carried out at the 
BuShips Test Station at Cape Henlopen, Delaware, 
in September 1944 and March 1945 between the 
shore station and the USS Marnell. The Emerson 
receivers were revised in accordance with these new 
developments with no change in type number. New 
type numbers were assigned to the other new equip¬ 
ments of this stage as follows: RCA 7 receivers 
were known as type US/D-1; the transmitting sys¬ 
tems were known as type US/D-2 with the power 
supply from Westinghouse Electric Corporation 


and the beacons from Portsmouth Navy Yard and 
the Naval Factory at Sommerworth, Maine. 

All the receivers in all three stages can receive 
from all the transmitters, but only the US/D-2 
transmitter and its laboratory type D-2 counter¬ 
part permit break-in from a distant station. 

Complete systems consisting of the US/D-2 
transmitters and revised Emerson US/D receivers 
were installed on two destroyer escort vessels in 
May 1945 and tested in the Pacific Theater of Oper¬ 
ations. The Navy reported some weeks later that 
the equipment “gave excellent results and met with 
wide acceptance.” 3 



L......... 

Figure 1 . Infrared source for ship-to-plane recog¬ 
nition equipment. Left to right: glass cover and 
filter, coding cylinder, modulating cylinder, cylindri¬ 
cal housing, motor assembly with tungsten lamp. 

General Design: Ship-to-Plane Recognition Sys¬ 
tem. The general design of the ship-to-plane system 
tested and demonstrated at Norfolk in February 
1943 is as follows. The 110-volt, 500-watt, T20 
tungsten projection lamp (see Section 1.2.2) 
mounted on the ship is coded at 87 cycles by a nine- 
bladed cylindrical inner shutter surrounding it, 
which is driven by a governor-controlled battery- 
operated 30-volt motor operating at 2,910 rpm 
(Figure 1). The opaque and transparent sectors of 
this shutter are equal. An outer shutter rotating at 
about 40 rpm has three adjustable sectors to pro¬ 
vide for transmitting any three-element code letter. 
The shutters extend well below the lamp, and their 
tops are cut away, except for a small central disk, 
so that the source transmits into the whole upper 
hemisphere except for a small cone of about 20 de¬ 
grees total angle directly overhead. 

The rotating shutters are enclosed in a 5-inch 
diameter, 9-inch high glass dome the inner surface 
of which is covered by a Polaroid XR7X25 filter 





174 


NIR RECOGNITION AND CODE COMMUNICATION SYSTEMS 


(see Crapter 2). A motor-driven fan ventilates the 
dome. 

The receiver mounted in the nose of the plane 
consists of a Cashman thallous sulfide (TF) photo- 
conductive cell with 1-inch square sensitive area 
(see Chapter 3). This cell is mounted axially at the 



Figure 2. Laboratory type D beacons No. 54 and 
No. 55. 


focus of a parabolic Alzak aluminum reflector, 
which is 9 inches in diameter and 1.7 inches in 
focal length, giving an 8-degree hpr angle. The 
photocell feeds into a fairly conventional two- 
stage preamplifier which in turn feeds a four-stage 
narrow-band amplifier peaking at 90 cycles with a 
12-cycle bandwidth. The total gain is over 150 db. 


A sensitivity control is provided. The output can 
be connected either to a neon indicator lamp or to 
headphones. When a transmitter comes into range 
in the field of view the lamp lights, or a code tone 
is heard. 

The ship transmitter weighs some 80 pounds, not 
including the 30-volt battery supply for the modu¬ 
lating motor, and it consumes about 500 watts from 
the 110-volt line. The plane receiver assembly 
weighs about 33 pounds including batteries and 
requires about 15 watts from its own batteries. 



General Design: Laboratoi'y Type D. In the 
second stage of development (ship-to-ship), revi¬ 
sions were made as indicated in Figures 2 and 3. 
Two transmitter beacons of a design similar to that 
in Figure 2 were to be placed on each ship so as to 
cover 360 degrees azimuth without any obstructions 
by the ship’s superstructure. Likewise, two receiver 
heads like that in Figure 3, one port and one star¬ 
board, were intended to be used so they would 
together scan 360 degrees in azimuth; but in the 
laboratory type D equipment built up for the USS 
Corry -USS Cormick tests in February 1944, only 
one receiver was actually constructed for each ship. 

The 500-watt transmitter lamps used at this stage 
are nominally of almost 1,000 cycles and were de¬ 
veloped especially for this project by the GE Lamp 
Development Laboratory at Nela Park, Cleveland, 
Ohio. The inner shutter around the lamp has three 
sectors and is rotated at 30 rps by a synchronous 
motor to give a 90-cycle signal. The outer shutter 
has been dispensed with, and a more flexible (but 
less crisp) coding is provided by interrupting the 





























































TYPE D SYSTEM 


175 


lamp current at the control panel. Any Morse letter 
may be repeated automatically and continuously, 
or the operator may send a message with a telegraph 
key. Either plain glass cylinders or marine Fresnel 
lenses may be used on the outside of the beacons; 
the latter confine the beam to about 22 degrees hpi 
vertical spread, as shown in Figure 4, giving an 



Figure 4. Distribution in vertical plane from labora¬ 
tory type D source in Corning Fresnel lens. 


optics factor of about 3X> or about 3,000 horizon¬ 
tal holocandlepower (see the Appendix for new 
nomenclature) when the high-altitude beams of 
ship-to-plane communication are not needed. A 
Polaroid filter is placed on the outer cylinder and 
on an inner glass cylinder such that the combined 
transmission corresponds to that of XR7X25. The 
double construction gives a reserve of security in 
case one layer cracks. 

An ingenious synchronization system keeps the 
sectors of the two beacons on a ship in exact direc¬ 
tional coincidence as they rotate so that the two 
signals are never out of phase. 

The laboratory type D receiver, shown in Fig¬ 
ure 3, consists of an optical head, training control 
box, amplifier, and indicator or headphones. The 
cell and reflector arrangement chosen results in the 
directivity pattern (Section 4.1.3) shown by the 
dotted curve in Figure 5, with an hpr width of 
about 11 degrees. The main difference from the 
earlier model is that each head is now placed on a 
vertical axle to scan automatically (or manually, 
from the control panel) back and forth through 290 
degrees. The scanning motor is governed by a servo¬ 
mechanism from the training control unit shown in 
Figure 3, and an indication of the bearing of either 
the port or the starboard unit at any instant to ± 1 
degree is provided on the main control unit, also 


shown in Figure 3. The scanning speed and coder 
speed can be regulated manually from the latter 
unit. 

The weights of the components of the laboratory 
type D, exclusive of cable, are: 


Main control unit 1701b 

Receiving heads (120 lb each) 2401b 

Training control 401b 

Sources (85 lb each) 170 lb 

Total weight, equipment for one ship 620 lb 


The power consumption is 1,000 watts for the bea¬ 
cons (when they are on about half the time), plus 
some 250 watts for the various motors and the 
receiver system, all supplied from the 115-volt, 
60-cycle ship supply. 



Figure 5. Directivity pattern of optical systems con¬ 
sidered for type D receiver. 


General Design: Laboratory Type D-2. In the 
final stage of laboratory development, the trans¬ 
mitter and receiver were both adapted for faster 
coding. The beacons were completely transformed, 
as shown in Figures 6 and 7. A thyratron supply 
(Figure 6) provides rectified half-wave pulses from 
the three-phase 60-cycle ship supply, selecting the 
pulses to give 90 cycles (see Figure 11). In the final 
model these pulses energize fifteen 10-watt, 230-volt, 






































176 


NIR RECOGNITION AND CODE COMMUNICATION SYSTEMS 



Figure 6. Transformer, thyratron control cabinet, 
and D-2 beacon. 

10S6 tungsten lamps (Figure 7) in each of eight 
reflecting troughs mounted in octagon array around 
the ship’s fighting bridge (Figure 8). These lamps 
cool so rapidly that the backscattered radiation 
from them does not deaden the local receiver in 
the intervals between signals as did the earlier 
500-watt lamps. The reply from a distant station 
can be heard, break-in, during these intervals. The 
faster response of the lamps also makes possible 
clear coding at speeds up to 40 words per minute 



Figure 7. Details of D-2 beacon. 


which are limited only by the skill of the signalmen, 
compared to maximum clear speeds of about 10 
words per minute with the older design. 

The eight beacons give 360-degree azimuth cov¬ 
erage, and the reflecting troughs are designed to 
give vertical hpi angles of 50 degrees. The mean 
horizontal intensity without filter is about 1,050 
cycles. Corning 2568 glass filters 8 millimeters 
thick have been used, but Polaroid XRN5PX65 
or the Ohio State University [OSU] DR 23u filters 




EIGHT 

BEACONS 

ARRANGED 

IN 

OCTAGON 

FASHION 

AROUND 

FIGHTING 

BRIDGE 


Figure 8. D-2 beacons installed. Eight beacons arranged in octagon fashion around fighting bridge. 

































TYPE D SYSTEM 


177 


are preferred because of their higher transmission 
(Chapter 2), now that they have passed Navy 
weathering and temperature tests. 

In this period of development the receivers, still 
designed to be two in number on each ship, are 
gyroscopically stabilized by impulses from the 



Figure 9. Type D-2 receiver head, cover off. 


main ship gyro (see Figure 9). The scanning indi¬ 
cator on the control panel (Figure 10) is linked 
with the ship's compass so that it will give both 
true and relative bearing of any source in the field 
of view. The analyzer stage of the receiver-ampli¬ 
fier has been changed so that it also serves as a 
voltage limiter and prevents the building up of the 
excessive voltages on the neon indicator lamp (from 
strong local backscatter) which had delayed lamp 
extinction on the earlier model and so would have 
prevented break-in reception even if the transmitter 
time lag had not been present. This voltage limiting 
feature actually permits narrowing of the pass band 
to 9 cycles while still permitting much faster coding 
speed. 


With the break-in system a distant station is 
detected when the neon indicator lamp gives signals 
which do not follow the local automatically coded 
source. After detection of a distant station, com¬ 
munication may be started by a “call-up" program 
in which the operator holds down his key continu¬ 
ously for several seconds. The neon lamp at the 
distant station then glows continuously for long 
intervals while the receiver is scanning in the gen¬ 
eral direction of the calling ship. 

At this stage of development a loudspeaker was 
added to the headphones and indicator lamp as a 
device for observing the signal. 



Figure 10. Type D-2 main control unit (Emerson). 


The weight of a complete laboratory model D-2 
ship installation is approximately as follows, exclu¬ 
sive of cable: 


Eight transmitter beacons 160 lb 

Two receiver heads, stabilizers, and training units 350 lb 

Control panel 170 lb 

Total for one ship 680 lb 


The power consumption is about 1,400 watts for the 
beacons (when they are on) from the 115-volt 
60-cycle three-phase ship supply, plus perhaps 250 
watts for the remainder of the equipment. 








178 


NIR RECOGNITION AND CODE COMMUNICATION SYSTEMS 


General Production Designs. The latest US/D-1 
receiver and US/D-2 transmitter designs follow 
quite faithfully the final revised laboratory model, 
the principal changes being to make the equipment 
rugged enough to pass Navy shock, vibration, tem¬ 
perature, and weathering tests. The Emerson US/D 
control panel is shown in Figure 10, and the D-2 
beacons shown in Figure 8 are also units available 
in the open market. The power consumption is, of 
course, about the same as for the laboratory model, 
but the weights and sizes are as follows (exclusive 
of cable): 


Unit 

Size Total weight 

Eight transmitter beacons 

Each 8x8x24 in. 

300 lb 

Two receivers 

Each 20x20x40 in. 

4801b 

Servoamplifier 

12x20x24 in. 

95 lb 

Motor generator 

8x12x20 in. 

125 lb 

Control panel 

18x24x28 in. 

195 lb 

Total for one ship 


1,195 lb 


Security. The desired night visual range [NVR] 
(see Section 4.1.3) of the assembled beacons on one 
ship was not to exceed 1,200 feet. The filters men¬ 
tioned above, which were used and recommended 
for the laboratory type D-2 model, give an NVR, 
from laboratory measurements, of 500 to 800 
feet. 

The ACW range of detection of the assembled 
D-2 beacons on one ship by a type C 3 or type C 4 
infrared electron telescope (see Section 4.1.3) is 
estimated from observations made during field tests 
to be not over 6 miles. The code message of the 
transmitters described above can, of course, be read 
with such a telescope at shorter ranges so that there 
is not even “message security” (see Section 4.1.2). 
At the request of BuShips, a modification of the 
transmitter was devised which gives message secu¬ 
rity. In this modification, instead of on-off keying, 
frequency-shift coding was tested, with the fre¬ 
quency changed by the key from a normal 180- 
cycle tone, to which the receiver is not sensitive, to 
the 90-cycle tone, to which it is. Arrangements are 
made to keep the average intensity the same at the 
two frequencies so that the change is imperceptible 
in an image tube. Unfortunately the pickup from 
backscatter when the transmitter is on continuously 
in this way is so large as to make break-in opera¬ 
tion impossible. Whether shielding of the receiver 
from the local transmitter and backscattering ob¬ 
jects can be devised so as to eliminate this difficulty 
is not known. There seems to be some radio broad¬ 


casting as a result of the thyratron pulses in the 
revised R-2 transmitters, but the seriousness and 
possible detection range of this is not known. 

Range. Although .extensive field tests have been 
made on the type D equipment in various stages of 
development, only one test seems to have been accom¬ 
panied by the measurements or estimates of atmos¬ 
pheric transmission which are necessary to stand¬ 
ardize the observed ranges (Section 4.1.3). This 
measurement 63 gives for the equipment of the 
second stage a range in vacuum of 20 sea miles with 
the Fresnel beacon, which would mean an ACW 
range of about 5 sea miles. The consensus of results 
with the type D-2 beacons indicates a vacuum 
communication range of about 30 sea miles. The 
ACW communication range is then about 6.5 sea 
miles; the ACW range for recognition may be about 
Y 2 mile greater. The ACW ranges would be in¬ 
creased nearly 1 mile by changing from glass filters 
to the plastic filters recommended. 

Evaluation. The final type D-2 design appears to 
fulfill very satisfactorily the specifications orig¬ 
inally set down for it. However, the D-2 beacons 
are two or three times as large and heavy as neces¬ 
sary to give the desired beam pattern. 

A comparison with the type E ship-to-ship voice 
communication system (Section 4.4.2) is interesting 
and instructive because the two developments ran 
parallel in many ways, and because after February 
1943 the communication function was emphasized 
for type D more than the identification function. 
For both systems, the final commercial installations 
built to Navy specifications weigh about 1,000 
pounds, a little more for type D, less for type E. 
Type D has a 360-degree transmitter, 11-degree 
automatic receiver; type E has a manually operated 
12-degree transmitter and 18-degree receiver. Type 
D ACW code range is about 6.5 sea miles; type E 
ACW voice range 6.5 sea miles, type E code range 
9 sea miles. Infrared telescope ranges are about 
9 miles for type E, perhaps 6 miles for type D-2, 
but type D-2 with break-in feature has no message 
security against telescope detection. Type D-2 con¬ 
sumes about 1,700 watts on, 250 off; type E con¬ 
sumes 1,100. Differences in range and power are 
the result of difference in transmitter angles and 
frequency bandwidth, as discussed in Section 4.1.3. 
Thus type E as a communication device has the 
obvious advantages of voice communication over 
code and has greater range (code) and more secu- 




TYPE D SYSTEM 


179 


rity than type D. The price of these advantages is 
the comparatively narrow angle of the transmitter, 
which makes it impossible, without some transmitter 
scanning system, for type E to be used at all for 
the identification function to which type D is very 
well adapted. 

Military urgency and production pressure, follow¬ 
ing the successful demonstrations of the early type 
D models, may have prevented adequate considera¬ 
tion of variant methods and may have led to prema¬ 
ture fixing of certain design features, so that it is 
not known at all whether the present type D-2 
models represent optimum design for a system of 
this type. For example, in the first stage of devel¬ 
opment the frequency was chosen near 90 cycles for 
the excellent reasons that it was mechanically con¬ 
venient and was not too close to the 60- and 120- 
cycle hum frequencies. Also this frequency was high 
enough to permit reasonable coding speed yet low 
enough to avoid the loss in sensitivity then mis¬ 
takenly thought to exist “because of the dropping 
response of TF cells at higher frequencies.” This 
90-cycle frequency was preserved throughout the 
development so that all models would be inter¬ 
changeable. It may not be an undesirable frequency 
now, if the question were looked into, but the orig¬ 
inal reasons for its adoption have almost disap¬ 
peared since the system is now electrically modu¬ 
lated and the TF cell threshold sensitivity is now 
known to extend almost unchanged to at least 3,000 
cycles. 

Against the 90-cycle frequency is the fact that 
the received code tone has so low a pitch as to make 
long attention unpleasant and tiring. To meet this 
problem two frequency-changers were finally de¬ 
signed, one by Contract NDCrc-185 and one by 
Emerson Radio, to bring the pitch up near 1,000 
cycles as an aid to the operator. Another possible 
objection to the low frequency is the interference 
from atmospheric “twinkle” effects (Section 4.1.3), 
which, it has been suggested, may be more serious 
here than at higher voice frequencies. Receiver de¬ 
sign also appears to be more troublesome than at 
higher frequencies. 

From the point of view of simplicity of power 
supply, there are good reasons for choosing 60 or 
120 cycles for the code frequency. 

As a result of the military urgency there also 
seems to have been no time for adequate considera¬ 
tion of cesium lamp sources at the time that the 


type D-2 electrically modulated beacon was being 
developed, although the tungsten lamps in this 
beacon appear to have a modulation ratio at 90 
cycles of the order of 30 per cent, while the cesium 
lamps have ratios near 100 per cent up to nearly 
10,000 cycles. 

Now that more leisurely and fundamental pro¬ 
grams are again possible it would seem that the 
whole question of the best frequency for a code 
system of this type should be restudied carefully, 
as has been suggested by those working under 
Contract NDCrc-185. 3 In such a review particu¬ 
lar attention should be paid to gaseous discharge 
sources. 

There was another unfortunate result of the mili¬ 
tary pressure. The parallel development of type D 
and type E, which was required because of the 
emergency, resulted in two commercial equipments 
of great weight and power consumption, the func¬ 
tions of which overlap a great deal but not enough 
to warrant eliminating either in a military situation 
requiring the NIR. Also, though they supplement 
each other, there may be difficulties in using them 
simultaneously on the same ship because of back- 
scatter and crosstalk. From this point of view it is 
fortunate that type D has such a low frequency in 
that the two frequency channels may be sharply 
separated by wave filters. Even so, the d-c com¬ 
ponent of backscatter will remain, causing great 
mutual losses in receiver sensitivity. If the type D 
frequency were raised, it would go no higher than 
about 400 cycles before the problem of crosstalk 
would become very serious indeed. 

As the coming of peace provides a breathing spell 
in which to reconsider the fundamental military 
needs, serious thought should be given to this whole 
problem of overlapping—and interference—of func¬ 
tions between recognition and voice communication 
systems. Considerable attention should be given to 
the possibility of amalgamating type D and type E 
into a single system which would preserve the best 
features and the most important functions of each, 
and effect a great saving in power and weight. More 
specific comments will be made below under “Rec¬ 
ommendations.” 

Regardless of these unresolved questions, type D 
has proved very successful in operation and has 
received enthusiastic recognition of its military 
value from Naval officers and from operating per¬ 
sonnel. 




180 


NIR RECOGNITION AND CODE COMMUNICATION SYSTEMS 


Transmitter 

Beacon Synchronizing Systems. The lamp sources 
and mechanical chopper designs used during the 
first two stages of development need no further 
description here except to mention that the trouble¬ 
some problem of motor lubrication at the high tem¬ 
peratures (up to 115 C) encountered in the beacon 
housing was successfully overcome. 

However, the use of two beacons on a single ship, 
in the second stage of development, introduced an 
interesting complication, namely the problem of 
developing a synchronizing system to keep the two 
rotating sectors parallel at all times. The 90-cycle 
modulation required that three-bladed shutters run 
in directional coincidence on separate four-pole 
motors. 

The mechanism devised by workers at the Univer¬ 
sity of Michigan under Contract NDCrc-185 to 
accomplish this involved having both motors rise on 
starting to full speed unsynchronized. Differential 
impulses from commutators on the two motors were 
used to actuate an auxiliary relay circuit. Then if 
the two motors were out of phase, the current 
through one motor was weakened by the relay until 
it “slipped” into directional coincidence with the 
other motor, at which time the relay ceased to oper¬ 
ate. The Crouse-Hinds Company substituted two- 
pole alternators on the motor shafts for the com¬ 
mutators. 

Coaxial Shutter. One interesting variant method 
of mechanical coding which was explored under 
Contract NDCrc-185 was the coaxial shutter. This 
shutter consisted of two ordinary three-bladed 
cylindrical shutters around the lamp, mounted one 
outside the other on a common axle and both rotated 
by the modulating motor at 30 rps. If the outer 
blades are aligned with the inner ones, the light 
comes through the three gaps and is chopped at 
90 cycles; a rotation of the outer shutter by 60 de¬ 
grees closes the gaps and cuts off the light. Nor¬ 
mally, a spring holds the outer one closed; keying 
applies a brake which drags on the outer one and 
opens it. 

Having a short time lag with such a system 
means having a small moment of inertia for the 
outer cylinder and using a stiff spring with a cor¬ 
respondingly high drag in the brake. With the 
motor used, this drag was sufficient to drop the 
frequency to 83 cycles when the brake was ap¬ 


plied, which is an intolerably large change; a 
stronger motor would be needed for successful 
operation. 

The reverse system of using the brake to close the 
shutter and the spring to open it would also be 
feasible. Braking may be accomplished either by a 
mechanical brake band or by eddy currents induced 
by an electromagnet. 

Such a device, if perfected, would allow break-in 
communication with rugged-filament lamps like the 
original type D beacons, since the coding would not 
be accomplished by interrupting the lamp current. 

Fresnel Lenses and Filters. The candlepower dis¬ 
tribution from the special 500-watt monoplane fila¬ 
ment lamps used in the first and second stages of 
development follows approximately the sine law, 
being greatest (almost 1,000 candlepower) in the 
horizontal plane and least in the vertical direction. 
The external clamps and supports for the beacons 
are designed to give minimum interference with the 
radiation, but nevertheless cause some weakening 
of the intensity in certain directions. The beacon 
shown in Figure 2 has its vertical beams obstructed 
by the opaque cap, since it was to be used only for 
transmitting to other ships. The lamp is mounted on 
a support rod through the hollow motor shaft so that 
if ship-to-plane communication were desired, the 
cap could be simply replaced by a filter glass giving 
a minimum of overhead obstruction. 

The filters used were Polaroid XR7X25 or the 
equivalent and gave an effective holotransmission 
[ehT] (see Chapter 2) of about 0.30 with the TF 
cells used in the receiver. 

Each beacon was supplied with two interchange¬ 
able glass housings, either a large plain cylinder to 
give the sine-law distribution noted above,' or a 
Corning marine Fresnel lens, type 53048-218J. This 
altered the vertical candlepower distribution as 
shown in Figure 4, giving an optics factor of 3X 
by cutting down the vertical spread of the beam to 
an hpi width of 22 degrees. The horizontal distribu¬ 
tion with the Fresnel lens remains equal in all direc¬ 
tions, except for losses due to obstructions as men¬ 
tioned. 

Control Panel. The control panel used in the 
second stage of development contained a special 
pilot light to show whether the beacons were prop¬ 
erly synchronized. It also provided either for send¬ 
ing a message by manual keying or for repeating 
automatically one or more code letters over and 



TYPE D SYSTEM 


181 


over. A motor-driven bank of cams which could 
make contact with a microswitch provided a choice 
of this automatic signal from among all the Morse 
letters. The frequency of the automatic coding, in 
letters per minute, could also be varied. 

The increased emphasis on the communication 
function in the third stage of development made 
some of these features unnecessary, as can be seen 
in Figure 10 showing one of the later control panels. 

Electrically Modulated Sources. The incandes¬ 
cence and nigrescence times of the large tungsten 
filament of the lamp used in the first two stages of 
development are a considerable fraction of a second. 
Thus even with slow coding at speeds under 10 
words per minute (about 1 letter per second) the 
radiation from the source is fairly strong through¬ 
out the off periods and swamps any faint signal 
from a distant source attempting to be heard. Also 
an attempt to code at much faster speeds results in 
fuzzy and confusing signals. 

These difficulties were remedied in the third stage 
of development of the type D-2 system by replac¬ 
ing the one large lamp in each beacon with 120 high- 
voltage (230 volts), 6-watt (6S6), and later 10-watt 
(10S6) tungsten lamps. These lamps have fine fila¬ 
ments which will go on and off very quickly. They 
are mounted in eight beacons for 360-degree azi¬ 
muthal coverage. Oscillograms of the radiation from 
these lamps showed that clear coding was possible 
at speeds up to 40 words per minute. This was con¬ 
firmed by actual tests at speeds up to 30 words per 
minute which is about the limit reading speed for 
most code operators. Since the lamps are on only 
intermittently, they can be and are operated at 
about 13 per cent power overload without any 
serious loss of operating life. 

Possible 90-cycle Power Supplies. The 90-cycle 
mechanical chopper arrangement, while workable 
with the large beacons, involved troublesome fea¬ 
tures of synchronization and mechanical mainte¬ 
nance which had already led to consideration of 
possible electrical modulation. For modulating the 
many small lamps, the devising of an efficient and 
trouble-free mechanical chopper would have been 
very awkward indeed, and electrical modulation 
became essential. 

Two possible systems for 90-cycle-per-second 
electrical modulation were considered but were not 
used. One consisted of a generator properly geared 
to a synchronous motor; procurement difficulties 


interfered with this arrangement. The second 
method involved the mechanical selection of alter¬ 
nate pulses from a three-phase 60-cycle Y system. 
In such a Y system the time between successive 
maxima is % 80 second, and between alternate 
maxima is % 0 second. The pulses could be selected 
by a suitable commutator and brush arrangement 
driven by a synchronous motor, but the problems 
of synchronization and phasing, adjustment of out¬ 
put power, and provision for keying promised to be 
difficult. 

Thyratron Power Supply. The system finally 
chosen involves a beautiful arrangement for elec¬ 
trical selection of pulses from such a Y system. The 
basic circuit consists of a set of transformers run 
from the three-phase supply, with the secondaries 
connected in a Y. Three thyratrons have their plates 
connected to the points of the Y and their cathodes 
connected together. The load is connected between 
the cathodes and the center point of the Y. 

The thyratron grids are normally negative, but 
are all made positive at the peak of alternate 
maxima by pulses from a 90-cycle oscillator. As 
Figure 11 shows, only one thyratron will then 
conduct, as the plates of the other two are negative 
for a shQrt time before and after the peak. The 
resulting pulses through the beacons are shown in 
curve F of Figure 11. The sharp current rise and 
the long off period during each cycle were found 
to give a much better 90-cycle light signal from the 
beacons than was obtained if they were simply run 
on sine-wave current, with the same mean power 
input in each case. 

The relaxation oscillator was synchronized by 
180-cycle pulses from a set of small phase-controlled 
thyratrons connected in a manner similar to the 
main power thyratrons. 

The bias on the power thyratrons is normally 
about 200 volts negative, and keying is accom¬ 
plished by raising the bias to about 60 volts nega¬ 
tive so the pulses from the oscillator will initiate 
conduction in the power thyratrons. 

The system is very stable with respect to line 
voltage changes. The output power can be adjusted 
for different input voltages and different loads by 
adjusting the phase of the oscillator pulses and so 
giving conduction in the power tubes for a longer 
or shorter fraction of a cycle. In the small thyra¬ 
trons the phase is varied either by adjusting the 
phase of a lagging a-c voltage on the grid or by 



182 


NIR RECOGNITION AND CODE COMMUNICATION SYSTEMS 


adjusting the potential of the shield grid. With the 
latter method, one voltage divider, connected to all 
three shield grids, will control the three phases to¬ 
gether. The correct power setting is shown to the 
operator by a monitor lamp on the control panel 
which should be neither brighter nor dimmer than 
a reference lamp located beside it. Correct thyratron 
operation is indicated by the 30-cycle flickering of 
each tube or of a small neon lamp in parallel. 



A VOLTAGES BETWEEN 3'PHASE LINES AND NEUTRAL 

L f\ r\ r\ r\ r\ r\ r 

B VOLTAGE ACROSS SYNC RESISTOR 

0 J_L_1_J_L_1_L 

C VOLTAGE APPLIED TO OSCILLATOR GRID 



D VOLTAGE ON OSCILLATOR CONDENSER 


. I_I i_L 

E VOLTAGE APPLIED TO GRIDS 10 


o-J— 


p\ f\ i\ r 


F OUTPUT VOLTAGE 


Figure 11. Voltages versus time in 90-cycle thyra¬ 
tron power supply, not to same scale. 


During installation of the D-2 transmitters for 
the tests in the Pacific, it was discovered that oper¬ 
ation of the power supply interfered considerably 
with radio communication. The Westinghouse-built 
power supply was redesigned with filters to elimi¬ 
nate most of this interference, at least on regular 
Navy radio-frequency bands. Further shielding of 
output and input lines and grounding of the cabinet 
may be necessary to make the system completely 
secure against enemy radio detection and reception. 

Inrush Keyers. At the request of Section 660E-9 
of BuShips, Contract NDCrc-185 made a study of 


methods for reducing the time lag of incandescence 
in large tungsten lamps used for code signaling, 
including the early type D beacon and others, such 
as the X2A used by the Navy. Nothing can be 
done about the cooling or nigrescence time of the 
filaments, but the heating or incandescence time can 
be shortened by overvolting the filament for an 
instant and reducing the voltage to normal as the 
current approaches normal. 

Two devices were built to accomplish this. One is 
the saturable core reactor keyer. In this the 500- 
watt beacon is placed in series with the two a-c 
windings of the saturable core reactor. The react¬ 
ance is large and little current flows until d-c cur¬ 
rent is made to flow in the third winding of the 
reactor. A d-c current of 1 ampere controls 10 
amperes a-c current. The initial overvolting of the 
beacon is accomplished by placing a 60-watt tung¬ 
sten lamp in the keying circuit, which thereby 
passes a 10-fold greater current when the lamp is 
cold than after it warms up. Suitable resistors and 
condensers in the d-c circuit control the time dura¬ 
tion of the overvoltage and prevent oscillation. 

The second device is the thyratron-controlled 
inrush keyer. In this system, the lamp current is 
phase-controlled by a thyratron full-wave rectifier 
of fairly conventional design. Closing the key, 
which is in the shield grid circuit, permits large 
current to flow, but starts a condenser charging 
which slowly changes the grid voltage. This permits 
conduction later and later in each cycle and reduces 
the power until the condenser is fully charged. 

The condenser discharges at about the same rate 
as it charges, which is presumably adjusted to the 
rate of lamp heating and cooling. Consequently, if 
the key is pressed before the beacon has cooled 
down, the overvolting is more gentle than when the 
beacon is cold. Otherwise, there might be danger of 
burning out the beacon by a series of quick starts 
with inadequate cooling in between. Probably the 
saturable reactor keyer has the same desirable be¬ 
havior because of the similar time lag of its control 
lamp and condenser in cooling down. 

A field test was made on both of these devices at 
the BuShips test station at Cape Henlopen on July 
31 and August 1, 1945. A beacon was keyed by 
these devices and by a direct keyer. The accuracy 
of reception was then compared for random code 
letter groups sent at various speeds and with the 
beacon at various distances from the receiver. These 



















TYPE D SYSTEM 


183 


devices gave a significant improvement in reception 
over the direct keyer, but there was some question 
as to whether the improvement would justify the 
added weight and cost. The true improvement pos¬ 
sible with these keyers may have been underesti¬ 
mated in these tests because of receiver limitations 
but exact data on this point are not available. 

The thyratron keyer would probably be the more 
flexible of the two systems for general use. 

Frequency Shift Coding. Because of the ease of 
reception of type D messages by image tubes at 
ranges comparable to the code range of the system, 
Contract NDCrc-185 considered other methods of 
operation which might give more message security. 
The most promising of these was a system in which 
the type D-2 beacons were normally operated con¬ 
tinuously at 180 cycles, a frequency to which the 
receiver is insensitive; the frequency would then be 
changed by keying, which would give a signal in the 
receiver as a code tone in the usual way. This sys¬ 
tem was especially simple with the thyratron power 
supply because it involved only the insertion of a 
15,000-ohm resistor in the cathode line of the syn¬ 
chronizing thyratrons to produce a 100- to 150-volt 
synchronizing pulse instead of the former 20-volt 
pulse and thus drive the relaxation oscillator at the 
frequency of 180 cycles. Keying closes a relay which 
shorts out this additional resistor, and the oscillator 
frequency drops back to 90 cycles. The beacon 
power at 90 cycles is adjusted by phase control, at 
180 cycles by control of the shield grid voltage. 
The power at the two frequencies can be brought 
to equality, so that keying produces negligible 
flicker in an image tube and the signal can no 
longer be read. 

This system was tested on August 14, 1945, at 
Cape Henlopen and gave ranges similar to those 
obtained by on-off keying of the beacons. However, 
the d-c component of the continuous backscatter 
from a transmitter was so strong as to make a 
nearby receiver insensitive to weak signals from a 
distant station at all times. As the preservation of 
the break-in feature was regarded by the Navy as 
more important than message security, this method 
of operation was therefore not adopted in the com¬ 
mercial equipments. Whether greater separation of 
the transmitter and receiver units on one ship and 
more optical shielding of them from each other and 
from backscattering objects would make break-in 
operation possible with this system is not known. 


Reflector Design. In the third stage of develop¬ 
ment it was desired that the transmitter beam have 
a vertical spread of 30 degrees above and below the 
horizon, so that communication would be fairly in¬ 
dependent of the roll of a ship. This distribution was 
achieved by placing a row'of 15 of the small lamps 
in a line in an Alzak aluminum trough as shown in 
Figure 7. The trough is made of seven plane strips, 
each of which reflects the light generally into the 
desired field. The horizontal distribution from a 
single trough unit follows roughly a cosine law, and 



Figure 12. Distribution of intensity of US/D-2 bea¬ 
con for various angles of roll. Expressed in percent¬ 
ages of intensity dead ahead on even keel. 


at least four such units are necessary for uniform 
360-degree coverage. Eight units were used in the 
actual type D-2 ship installations. The mean hori¬ 
zontal candlepower of this array, with the usual 
13 per cent overload on the small lamps, is about 
1,050, and the signal is thus somewhat stronger 
than that of the type D beacons in a plain housing. 
This distribution in the horizontal (geographic) 
plane for various angles of roll of a ship is shown 
in Figure 12. 

This trough design is two to three times larger 
than necessary for obtaining this distribution and 
candlepower. A saving in weight of 100 to 200 
pounds in the commercial US/D-2 installation 
could be achieved by a slight redesign of the units. 

Filters. The filters used in the first two stages of 
development were the Polaroid XR7X25 plastic 
sheets which had ehT values of about 0.30 for the 





184 


NIR RECOGNITION AND CODE COMMUNICATION SYSTEMS 


TF cell receiver and which gave an NVR of the 
order of 50 feet or less for the two 3,000-hcp Fres¬ 
nel beacons of stage 2. 

In stage 3, Corning 2568 filters were used for 
actual installations as they were at first the only 
ones which would pass Navy weathering tests. In 
8.5-mm thickness, they give an NVR for a com¬ 
plete type D-2 ship installation (eight beacons in 
octagon array) of about 400 feet, and an ehT for the 
TF cell of about 0.30. With 7.4-mm thickness, the 
NVR approaches 1,200 feet, ehT 0.32. Now that 
Polaroid PVA filters and Ohio State University 
[OSU] filters have passed the weathering tests, they 
are recommended for use on the type D-2 beacons. 
With thicknesses and dye concentrations giving an 
NVR near the limit of 1,200 feet desired for type 
D-2, these filters have TF-cell ehT values between 
0.55 and 0.60, which corresponds to an increase in 
ACW communication range of the equipment of 
almost 1 mile over the range obtainable with the 
Corning filters. 

Receiver 

Cell Arrangement. The photodetectors used 
throughout the type D development have been the 
Cashman TF cells with about %x%-inch sensitive 
grid area (type A cells). Indeed, these cells were 
designed and constructed by Northwestern Univer¬ 
sity under Contract OEMsr-235 almost entirely to 
meet the specifications and requirements of the 
type D development (Section 3.3.1). Various cell 
and mirror arrangements have been tried but the 
one actually adopted in all stages of development 
has been the mounting of the cell with its grid 
horizontal at the focus of a 9-inch diameter, 1.7- 
inch focal length parabolic Alzak aluminum re¬ 
flector (an auto headlight mirror), as shown in 
Figures 3 and 9. With the present type A cell de¬ 
sign, the cell is mounted parallel to the axis of the 
mirror, with its base in a socket at the vertex of 
the mirror. The horizontal directivity pattern (Sec¬ 
tion 4.1.3) from this arrangement is shown in 
Figure 5; the vertical pattern is similar. The hpr 
width is about 11 degrees. 

The directivity patterns for the same cell in some 
larger mirrors are also shown in Figure 5. As ex¬ 
pected from the discussion in Section 4.1.3, the use 
of the larger mirror area increases the sensitivity, 
but decreases the hpr angle of view; or, to say it 
another way, the response with the larger mirrors 


(about 18-inch diameter) is better throughout the 
central 10-degree cone, worse outside. 

In spite of considerable changes in the main 
function and manner of use of the whole system, the 
cell and mirror arrangement has been kept almost 
fixed throughout the development; this was done 
partly in order to facilitate production. If this work 
is to be carried farther, as now seems likely, one of 
the first points to be reconsidered might be whether 
the cell and mirror arrangement is still the best for 
the present purpose of the system. Gyrostabiliza- 
tion has eliminated the need of the large vertical 
angle, if only ship-to-ship communication is in¬ 
tended; a narrower horizontal angle of view might 
be better. The acceptance of narrower angles in 
both dimensions would greatly increase the attain¬ 
able sensitivity and range (see Section 4.1.3). In 
particular, two TF cells of sizes quite different from 
the present type D size are now available. With the 
present mirror, use of the present ^x^-inch cell 
(see Chapter 3) would give about 1.5 miles more 
ACW range than the type A cells, and would give 
hpr angles of 4 degrees. Use of the l 1 / 4x2-inch 
(type B) cell would give about 0.3 miles less ACW 
range than type A cells, but would give hpr angles 
of about 25 degrees. With the proper 18-inch diam¬ 
eter mirror, all these ranges would be increased 
about 1.5 miles, hpr angles cut in half. 

Mechanical Operation. The receiver heads are 
driven by reversing motors for automatic scanning 
or for manual scanning in a fairly conventional 
manner. In the second stage of development the 
scanning speed could be adjusted at the control 
panel, but this feature was later eliminated as being 
unnecessary. Microswitches limit the travel of each 
head and reverse the scanning motor. The motor 
may be stopped and, by means of a selsyn arrange¬ 
ment, the beacon may be pointed manually in any 
direction by the operator at the control panel. A 
selsyn operated by the orientation of the heads can 
be used to drive the bearing indicator on the control 
panel so that it gives a continuous indication to the 
operator of the direction in which either the port or 
starboard receiver is pointing, as desired. The 
accuracy of these selsyns and the sharpness of the 
central peak in the receiver directivity pattern is 
such that bearing of a distant station can be 
obtained to within 1 degree of arc. In the final 
model signals from the ship’s gyrocompass were 
sent to a second bearing card within the bearing 



TYPE D SYSTEM 


185 


indicator so as to give true bearing as well as rela¬ 
tive bearing (Figure 10). 

The receiver heads in the first type D equipment 
were mounted on a vertical axle which swung with 
the roll of the ship (Figure 3), but in the final 
model, type D-2, impulses from the ship’s gyro- 
stabilizer drive a motor which keeps the axis of 
rotation accurately vertical and the receiver pointed 
accurately at the horizon (Figure 9). 

Preamplifier. In the first phase of development, 
a conventional two-stage preamplifier was used, 
having a gain of 48 db. Later the first stage of the 
preamplifier was made a cathode-follower stage just 
as in the TF-cell preamplifiers used in the type E 
development ’(Section 4.4.2). In the second stage of 
development, one of the two tuned circuits used 
was incorporated in the preamplifier, but this was 
removed again in the last phase of development. 
The preamplifier box of the laboratory type D is 
seen in Figure 3; in type D-2 the preamplifier itself 
is mounted behind the receiver head, as seen in 
Figure 10. 

Amplifier. In the airplane receiver used in the 
first stage of development the main amplifier had 
two tuned and two untuned stages. Several variants 
were later produced, including a power amplifier 
designed especially for operating headphones above 
aircraft noise levels. 

All have as their function the amplification of the 
received code tone to the point where it will operate 
either a neon indicator lamp on the control panel 
or a pair of headphones, or a loudspeaker. 

The narrow band-pass, about 10 cycles wide, 
peaking at 90 cycles, at first was obtained by two 
single-frequency resistor-capacitator analyzer cir¬ 
cuits tuned to slightly different frequencies, 87 
cycles and 93 cycles. Each circuit worked into a 
negative-feedback amplifier stage. In the type D-2 
system the use of the overload limiter made it pos¬ 
sible to narrow the frequency band somewhat, elimi¬ 
nating one of the tuned circuits. 

The amplification was controlled at first by a 
step attenuator on the grid of the first tube in the 
amplifier, because it was feared that a continuously 
variable potentiometer of the usual type would be 
noisy on aircraft or on shipboard. Later a suitable 
potentiometer was found for this control. 

A rectifier-filter unit supplies 115 volts d-c for all 
filaments so that the receivers can be run from 
either alternating or direct current. 


Output Limiter. The bandwidth of about 10 
cycles was adequate for a code speed of about 
8 words per minute, as used in the first type D 
beacons. This bandwidth, in the conventional linear 
circuits used at first, gave the receiver signal a rise 
and decay time lag of the order of % 5 second. 
Figure 13 shows the receiver time lag in an oscil¬ 
logram of the response to the letter F sharply coded 
at 5.5 words per minute. This lag was not objection- 



j 


Figure 13. Oscillogram of type D receiver response 
to the letter F, sharply coded at 5.5 words per minute. 

able, as it was no greater than the time lag in the 
filaments of the 500-watt beacons. Indeed, with the 
receiver at maximum sensitivity, it would continue 
to pick up backscattered radiation from nearby 
beacons for a period of several seconds after they 
were turned off. But when the change was made to 
the final small-filament, short time-lag, type D-2 
beacons to permit break-in operation, it was also 
necessary to reduce the time lag of the receivers to 
correspond. 

As the attainable speed of the new transmitter 
was of the order of 40 words per minute (greater 
speeds are useless because of the human limitations 
of the operator) the new receiver speed had to be 
comparable. Such speeds, with Morse letters, re¬ 
quire time lags of the order of % 0 second or less, 
and the receiver band-pass would conventionally 
have to be about 40 cycles or more wide to achieve 
such a fast response. Such a width would have 
greatly increased the noise voltage over that ob¬ 
tained with, the 10-cycle bandwidth, and thereby 
greatly reduced sensitivity. 

This impasse was ingeniously surmounted. A 
voltage-limiter stage was introduced in the receiver 
before the analyzer stage so that the voltage level 
could never rise more than 2 db above the value 
necessary to light the neon lamp. Thus the voltage 


tmt i r u 

Tfr^ nUuHTO 







186 


NIR RECOGNITION AND CODE COMMUNICATION SYSTEMS 


had only to rise 2 db above threshold to give maxi¬ 
mum lamp signal, and had only to fall 2 db from 
maximum in order to extinguish the lamp. Since the 
usual response time is reckoned as the time for 
voltage to change by about 9 db, the neon lamp is 
lighted or extinguished in an interval equal to about 
one-fourth of the response time. With the 10-cycle 
bandwidth, which has a response time of the order 
of y 10 second, a time lag of % 0 second for the neon 
lamp is achieved using the limiter system. Thus the 
desired increase in code reception speed by a factor 
of 4 is obtained with no increase in bandwidth or 
loss in sensitivity. Actually a slight narrowing of 
bandwidth was introduced in the final receivers 
without any serious effect. The output of the limiter 
is made linear up to and slightly beyond the point 
where the neon lamp is excited, so that the threshold 
sensitivity of the receiver is unaffected. The limiter 
introduces distortion of the output which is notice¬ 
able in headphones but does not interfere with the 
intelligibility of the received signals. 

Control Panel: Presentation. Control panels at 
the type D and type D-2 stages of development 
are shown in Figures 3 and 10. The signal neon 
light is shown in Figure 3 in the upper right-hand 
corner. In the equipment shown in Figure 10, the 
port and starboard indicator lamps are on each side 
of and just above the bearing indicator dial. The 
handle for manual scanning is below the bearing 
indicator. 

In operation, the gain control is normally set so 
that when the local beacon is off occasional noise 
pulses light the neon lamp, but not enough to 
confuse such a response with a distant signal. The 
coding on the local transmitter will then in general 
be repeated faithfully by the lamp until the receiver 
scans across a distant source. Then the code pat¬ 
tern will be altered or the neon lamp will glow con¬ 
tinuously until the receiver has scanned across the 
source. The general location of the distant source is 
determined by observation of the bearing indicator 
at the part of the scanning cycle where these irreg¬ 
ular signals occur. Call-up and communication may 
then proceed as described previously under “General 
Design: Laboratory Model Type D-2.” 

The headphones are about 10 db more sensitive 
and give ACW ranges about 1.5 miles greater than 
the neon lamp. For threshold signals the head¬ 
phones operate best with the lamp turned off. 

The use of a call bell or horn to give an audible 


indication of the presence of an incoming signal is 
highly desirable and would release the operator for 
other work. With such a bell, arrangements would 
have to be made to deaden the receiver during the 
instants that the local transmitter is on, so that 
backscatter would not produce calls. For maximum 
sensitivity, occasional false “noise” signals on the 
bell would have to be tolerated, but these would 
probably be less tolerable than the false flashes on 
the neon lamp and thus there might be a loss in 
sensitivity of about 10 db from that of the neon 
lamp presentation, corresponding to a further loss 
in ACW range of about 1.5 miles. 

Signal Frequency Changers. Although there is no 
loss in range occasioned by the poor tone quality 
of the 90-cycle signal in the receiver, operators 
accustomed to radio reception object to this quality. 
One method of increasing the frequency to give a 
more pleasing pitch would be to connect the neon 
indicator lamp as a relaxation oscillator and amplify 
the resulting signal for headphones or loudspeaker. 
Another method is used in the Emerson radio¬ 
frequency changer which has a continuously operat¬ 
ing 1,000-cycle oscillator connected to the head¬ 
phones through a “trigger” tube by the receiver 
90-cycle signal. 

It may be noted here that some experiments were 
made under Contract NDCrc-185 on a receiver for 
unmodulated NIR radiation, which operated by 
transforming such radiation into a 300-cycle code 
signal, but the results did not appear to be promis¬ 
ing for shipboard use. (However, the JAPIR air¬ 
craft system in Section 5.5 can perform this func¬ 
tion.) 

Test Beacons. Two different kinds of microflux 
sources or microbeacons for testing the operation 
of receivers were developed under Contract NDCrc- 
185 during the type D development. They have 
already been described in Section 4.1.3, as they are 
usable with all types of NIR receiving systems. 

Operational Tests 

Ship-to-Plane. Identification of a ship by a plane 
was tested with the equipment in the first stage of 
development on the nights of February 9 and 10, 
1943, and was demonstrated to representatives of 
BuAer, BuShips, and other Army and Navy officers 
on February 12, 1943, at Norfolk, Virginia, with 
the cooperation of the Navy and Naval Air Force. 1 
The 500-watt beacon (Figure 1) was installed above 







TYPE D SYSTEM 


187 


the wheelhouse of the coastal patrol yacht PYC26. 
The receiver head (14 degrees hpr width) was fixed 
pointing straight ahead in the bombardier’s com¬ 
partment in the nose of a PBY5 amphibian plane. 

On February 9 the PYC26, anchored near the 
mouth of Chesapeake Bay, and the PBY5 made 
several straight runs from various directions head¬ 
ing toward the ship. The source on the ship was 
operated continuously and the coded signals were 
picked up at distances of about 3.5 miles. On 
February 10, with the ship anchored near Cape 
Charles in a spot with less traffic and fewer shore 
lights, detection was obtained at distances esti¬ 
mated as up to 5 miles. No attempt was made to 
determine maximum range. 

February 12, the night of the official demonstra¬ 
tion, was relatively clear. One flight was spoiled 
because of blocking of the beam by the ship’s 
smokestack owing to a poor choice of flight direc¬ 
tion. On the other flights, ranges estimated at over 
5 miles were obtained. The pilot was able to use the 
apparatus as the sole means of locating the ship, 
and it appeared that he could have kept his bearing 
on the ship solely by observation of the indicator 
lamp. 

No difficulty was experienced from vibration, but 
the amplifier used was not powerful enough to give 
a signal in the headphones over the plane noise. 
Later a power amplifier was constructed for this 
purpose. 

It was found that 60-cycle shore lights also gave 
signals in the receiver. 

Following these tests the equipment was turned 
over to BuShips for field tests during which a range 
of over 4 miles was obtained on a land station. 

Solomon’s Island Tests. Under Project Control 
NS-151 BuShips then requested the construction of 
six automatic receivers and two modulated trans¬ 
mitters. The improved and modified models of this 
second stage of development were tested between 
ships near Solomon’s Island, Maryland, early in 
July 1943. Ranges up to 9 miles were attained on 
a night of better than average visibility. 

In these tests the receiver was mounted on a 
tripod and could be rotated at 2, 4, 6, and 10 rpm. 
Coding speeds were variable from 5 to 15 words 
per minute. A scanning speed of 5 rpm and coding 
speed of 10 words per minute appeared to be best. 

These tests were so successful that BuShips re¬ 
quested construction of two more complete sets, 


each with two transmitters and one receiver, rugged 
enough for shipboard installation and tests under 
simulated operating conditions. 

USS Carmick and Corry Tests. At the end of the 
second stage of development, the beacons, receiv¬ 
ers, and control panels were jas shown in Figures 2 
and 3. This equipment was tested on two destroyers, 
the USS Carmick and the USS Corry, about 40 
miles off the coast near Norfolk, Virginia, on the 
nights of February 7 and 8, 1944. 2 > 3 One ship could 
call the other, then the automatic scanner on the 
receiving ship could be stopped and messages ex¬ 
changed by code. The relative bearing of a trans¬ 
mitting ship could be obtained correctly to within 
about 2 degrees. 

The two beacons, port and starboard, on each 
ship gave a total of 360 degrees coverage. The single 
receiver on each ship scanned only 270 degrees. 

Receiver vibrations interfered with determination 
of maximum range on the first night. On the second 
night, a very clear night with bright full moon and 
calm sea, ranges up to 6.5 sea miles were obtained 
with plain beacons, up to 13 miles with the Fresnel 
lenses. 

Code speeds of about 10 words per minute were 
felt to be the best. Receiver threshold angles of view 
were determined by various ranges; values decreas¬ 
ing from 24 to 12 degrees were obtained, with range 
increasing from 3 to 7 miles. 

On the second day daytime signaling was at¬ 
tempted with no success even at ranges as short as 
1,000 yards. 

There was some interference of one beacon with 
the TBS radio. A 21-inch NIR searchlight was 
easily detected by the receiver at 2 miles. The 
officers and personnel reacted favorably to the 
equipment, noting especially that little special train¬ 
ing was needed to operate it. There was no objection 
to the bulk or weight, but the development of 
break-in operation was highly recommended. More 
powerful lamps and larger reflectors were suggested 
for increasing the range. A true bearing indication 
was recommended. 

The success of these tests led to Navy orders 
being placed (with manufacturers) for ten trans¬ 
mitters, each having two beacons and one control 
panel and automatic keyer; and for ten receivers, 
each having two heads, one control panel, and one 
servoamplifier—all to be suitable for permanent 
ship installation. 



188 


N1R RECOGNITION AND CODE COMMUNICATION SYSTEMS 


Cape Henlopen Tests. The first field tests on the 
laboratory model type D-2 system, as designed for 
break-in operation, were made at Cape Henlopen 
on September 5 and 6, 1944. The fine-filament 
sources and the receivers were operated at 60 cycles 
to simplify the tests of the feasibility of the new 
design. Communication was carried out between the 
USS Marnell and the shore station. The tests were 
only qualitative in nature but proved satisfactory, 
and work was immediately started on a more 
rugged model of the beacon and on the development 
of the 90-cycle thyratron power supply. 

Further field tests were made at Cape Henlopen 
with the final laboratory type D-2 systems on 
March 19, 20, and 21, 1945, again between the USS 
Marnell and the shore station. Signaling speeds of 
30 words per minute were recorded, the speed being 
limited by the skill of the operator rather than by 
the performance of the equipment. Break-in opera¬ 
tion was satisfactory even at threshold ranges. 

No attempt was made in these tests to determine 
maximum operating ranges, but readable signals 
were received at distances up to 6 sea miles. 

Pacific Tests. The latest type D-2 equipments 
were installed by BuShips aboard two destroyer 
escort vessels in May 1945 for tests in the Pacific 
Theater of Operations. The tests were started early 
in June 1945 and continued for several weeks, with 
the general report being made by the Navy that the 
equipment “gave excellent results and met with 
wide acceptance.” 

Other Tests. Some other field tests on related 
apparatus have been mentioned above under 
“Inrush Keyers” and “Frequency Shift Coding.” 

Present Status 

The equipment and facilities and part of the 
personnel of Contract NDCrc-185 at the University 
of Michigan have been transferred to a Navy con¬ 
tract with the same institution to continue this and 
related work. Quantity production contracts have 
been placed by the Navy with several manufac¬ 
turers for both type D and D-2 systems. 

Recommendations 

The time seems ripe for reconsideration of a num¬ 
ber of type D features, from the point of view of 
type D operation alone. 

First, the low 90-cycle frequency may be a dis¬ 
advantage, now that the practical factors in its 


initial adoption have been modified by later devel¬ 
opments. Work has already been started by BuShips 
on higher frequency identification sources, such as 
high-power cesium lamps. Other gas discharge 
tubes should also be studied, also methods for 
coding them efficiently and for obtaining message 
security. Those working under Contract NDCrc-185 
have also recommended study of 60-cycle and 120- 
cycle beacons because of the saving in weight on a 
power supply for them. 

Second, the present US/D-2 beacon design is 
larger and heavier than it needs to be, as mentioned 
above under “Reflector Design.” Some thought was 
given to reducing its size and weight during the 
development, but a great improvement still seems 
possible. 

Third, the present receiver directivity patterns 
may no longer be especially well suited to their 
function, and a change in cell size or mirror size or 
both may be in order. Those working under contract 
NDCrc-185 propose saving weight by using a wide- 
angle receiver without the gyrostabilizer. 

Fourth, the use of lead sulfide (PbS) cells (Chap¬ 
ter 3) for reception in the HR must now be con¬ 
sidered as offering a new identification channel. 

Further recommendations must be made if type 
D is to be operated in the presence of other NIR 
systems. 

If the Pacific tests did not include tests on the 
mutual operation of and interference between type 
D and other shipboard infrared devices, such tests 
should be carried out in the near future. Systematic 
studies of this kind were recommended in a BuShips 
report Cb as far back as March 1944. If they have 
actually been made, no report has been received 
of them. 

Since work on both type D and type E systems 
is being continued by the Navy, it is most impor¬ 
tant that designs, functions, and call-up and com¬ 
munication procedures be jointly and immediately 
reconsidered as a basis for revising them to reduce 
the mutual interference if installed on the same 
ship, or, better, amalgamating them into a single 
system as suggested above under “Evaluation.” 

If interference between type D and type E on 
the same ship is to be avoided, the following rules 
must be followed. First, the type D code frequency 
must be well out of the critical voice band from 
1,000 to 2.000 cycles if the voice receiver and code 
transmitter are to be operable simultaneously, or 




PLANE-TO-PLANE RECOGNITION [PR] SYSTEM 


189 


vice versa. Type D must stay below 400 cycles or 
go above 3,000 cycles if the present type E is to 
function at all on the same ship. Second, even with 
frequency separation, the d-c part of the back- 
scatter will reduce the sensitivity of both receivers 
unless transmission and reception are sharply sepa¬ 
rated further either (1) in time, as by a send- 
receive switch, (2) in space, as by accurate direc¬ 
tional beaming and shielding of receivers, or (3) 
in wavelength, as by use of the NIR for one func¬ 
tion and the HR for the other, or (4) in some com¬ 
bination of all these ways. 

As for amalgamation of the two systems, several 
things have to be considered—their functions, nec¬ 
essary beam and view angles, the methods of 
communication, and perhaps new experimental op¬ 
erating data on backscatter—before detailed rec¬ 
ommendations can be made. It may not be feasible 
to have the same source beacons for both voice 
and all-round code because of power and modulation 
requirements. If this is so, an amalgamation might 
still be made of the receiving heads and of the 
control panels, with a possible saving of several 
hundred pounds for a single shipboard installation 
over the combined weight of both systems. A nar¬ 
row-beam voice system with compact optical head 
like the RCA type G (see Section 4.3.1) would 
coalesce very easily indeed with the stabilized type 
D receiver. 

53 PLANE-TO-PLANE RECOGNITION 
[PR] SYSTEM 

Description and Performance 

Course of Development. In June 1944, Section 
16.4 of NDRC proposed to several different 
branches of the Armed Services the development 
of an NIR plane-to-plane identification system sim¬ 
ilar to type D (Section 5.2) but lighter in weight. 
BuAer took up this proposal and as a result Project 
Control NA-194 was assigned to those working 
under Contract NDCrc-185 to develop type D. 
The specifications were for the development of all¬ 
round (spherical) transmitters and for receivers 
with fore-and-aft coverage and some directional 
indication. The ACW range was to be over 2,000 
yards, NVR less than 100 feet, weight less than 25 
pounds, power less than 240 w T atts. For plane-to- 
ship identification, which was also wanted, the ACW 
range to and from type D was to be 12,000 yards. 


Presentation of the signal in a gunsight, as a warn¬ 
ing against firing on friendly planes or ships, was 
desired. 

Not all these requirements were accepted by Sec¬ 
tion 16.4 or met in the unit developed (the PR 
system) . 9 Studies were made; with direction-indicat¬ 
ing receivers. The receiver adopted for the first 
model is simply a stationary forward-pointing unit 
(for fighter planes) similar to that of type D with 
an hpr angle of view of 15 degrees. 

This development was carried out under Project 
Control AC-101, requested by the Army Air Forces, 
in addition to Project Control NA-194, since AC- 
101 requested the development of an identification 
system with essentially similar objectives. 

Three complete sets of the equipment, ready for 
installation in airplanes, were shipped in July 1945 
to the Patuxent River Naval Air Station for opera¬ 
tional tests which had not yet been carried out in 
January 1946. 



Figure 14. Nacelle beacon (PR system). 

General Design. The sources on each plane are 
four 6-watt, 115-volt type 6S6 tungsten lamps 
mounted one above and one below each wing tip in 
small nacelles of NIR filter glass as shown in Fig¬ 
ure 14. At least one lamp can be seen from every 



190 


NIR RECOGNITION AND CODE COMMUNICATION SYSTEMS 


direction. The lamps are operated from the plane 
115-volt, 800-cycle supply, interrupted by a 90- 
cycle commutator and a mechanical coding disk 
(Figure 15) which repeats a single call letter about 
25 times per minute. 



11 n 11 1 • M i * ! i i i i . n . iii i * i i * i 

! 2 3 .4 5 6 7 8 9 10 II 


Figure 15. Interior of PR modulator-coder unit. 

The receiver consists of a type A TF cell mounted 
axially in an SV^-mch diameter, l^-inch focal 
length Alzak aluminum reflector, as shown dis¬ 
assembled in Figure 16. This gives an hpr width 



Figure 16. PR preamplifier with reflector removed. 

of 15 degrees. The cell feeds into a cathode fol¬ 
lower and one stage of amplification in the pream¬ 
plifier mounted directly behind the mirror. The 
signal is then carried over a shielded cable to the 
amplifier, which introduces another stage of ampli¬ 


fication before applying the signal to a neon 
indicator lamp mounted in a Polaroid dimming 
socket on the instrument panel of the airplane. The 
frequency response curve peaks at 90 cycles with 
a width of about 6 cycles. 

The total weight of the equipment is about 25 
pounds. It requires about 35 watts of power from 
the 800-cycle, 115-volt a-c aircraft supply (most of 
which is consumed by the transmitter lamps) plus 
about 53 watts from the 28-volt d-c supply. 

Security. The Corning glass nacelle filters are ap¬ 
proximately equivalent to 2.4 mm of Corning 2540 
filter and give an NVR less than the 100 feet 
specified. 

The ACW range to a type C 3 or C 4 infrared 
electron telescope is estimated from experience with 
type D and type E to be about 3,000 yards. 

Range. The vacuum range computed from lab¬ 
oratory measurements is 4,000 yards in a direc¬ 
tion from the plane in which only one of the 
sources is visible; the ACW range is then 2,800 
yards. For directions in which two or more sources 
are visible, the range is correspondingly increased, 
becoming about 3,500 yards ACW for two sources, 
for example. 

Evaluation. The system fills the major require¬ 
ments specified in Project Control NA-194 except 
for the request for some directional indication. It 
was the conclusion of those working under Con¬ 
tract NDCrc-185 that this could not be obtained 
without a great increase in complexity and weight 
and a loss in range. It is considered that the re¬ 
ceiver could be mounted on gimbals (in a patrol 
bomber) so as to be electrically pointed with the 
guns and give a gunsight indication of a received 
signal, provided the necessary additional weight 
could be tolerated. 

The possibility of scanning a field for directional 
indication was eliminated because of the great time 
lag of TF cells and consequent slow scanning re¬ 
quired. It was recommended that PbS cells be 
studied in this connection, as the latter have short 
time constants. Of course, the tungsten lamps could 
not be efficiently modulated at the higher fre¬ 
quencies required for fast scanning and would have 
to be replaced by, say, some modification of the 
experimentally developed 20-watt cesium lamps 
(see “Recommendations,” Section 4.4.2) which give 
200 per cent light modulation efficiency (see Table 
2 of Chapter 4) up to 5,000 cycles. It would seem 








PLANE-TO-PLANE RECOGNITION [PR] SYSTEM 


191 


that the question of scanning and directional indi¬ 
cation with a PR system should be considered fur¬ 
ther in connection with the other Navy specifica¬ 
tions on the equipment. 

It might be pointed out that the ranges predicted 
here are of the same order as those expected at 
night for the voice plane-to-plane communication 
(P-P) system discussed in Section 4.4.4, although 
the very wide-angle reception of the latter—120 
degrees fore and aft, with the receivers used—re¬ 
duces sensitivity so that much more power (500 
watts) and weight (100 pounds) are required. One 
question might be whether the P-P and PR systems 
could not, for many purposes, be combined in a 
single system which would perform both functions. 

Transmitter 

Sources. Because of thermal lag in the filaments, 
the modulation at 90 cycles is not complete. The 
signal strength of one of the 6-watt lamps is equiv¬ 
alent to that of a 4-candlepower tungsten lamp with 
its beam chopped by a rotating sector to give a 
square-wave pattern. 

The 90-cycle coding commutator is on the shaft 
of a governor-controlled 2,700-rpm 28-volt d-c 
motor (Figure 15). The coding disk is run from the 
same motor through a reducing gear. 

Receiver 

Preamplifier. The preamplifier is mounted imme¬ 
diately behind the receiver mirror, as shown in Fig¬ 
ure 16. An interesting feature is the use of another 
TF cell of matched resistance as the load resistor 
for the actual receiving cell. This keeps the resist¬ 
ances balanced for the low temperature encoun¬ 
tered at high altitudes where the cell resistance may 
rise as high as 100 megohms as compared with about 
10 megohms for room temperature. The use of 
a reactance load has been proposed in the P-P sys¬ 
tem to meet this problem of the change in re¬ 
sistance, whether with temperature or with back¬ 
ground light (see “Preamplifier” under Section 
4.4.4). 

The cathode-follower stage does not amplify the 
signal but does amplify the microphonics due to 
plane vibration. The use of a thermostatically con¬ 
trolled heater for the cathode-follower tube has 
been considered for reducing the noise from this 
source (see also “Preamplifier” under Section 5.5 
for other methods of eliminating vibration noise). 


After the cathode-follower stage, the amplifi¬ 
cation stage feeds into a low-impedance output 
stage to permit transmission over a shielded cable 
to the amplifier. 

Amplifier. The amplifier contains a gain control, 
an analyzer stage, and a fipal output stage. 

The power supply for the TF-cell and amplifier 
plate voltage on two of the units constructed con¬ 
sists of a full-wave transformer-rectifier and filter¬ 
ing circuit run from the 800-cycle 115-volt supply. 
On the third unit constructed, a voltage-doubling 
circuit is used without any transformer. 

The threshold of the equipment is limited entirely 
by the TF-cell noise which is considerably greater 
than the noise produced by the amplifier used. 

Backscatter. Since, for identification purposes, 
the lamps and receiver must all be operated contin¬ 
uously, backscatter from the local coded sources may 
seriously limit the sensitivity to distant sources. 
Night tests were made during a light haze with re¬ 
ceiver and sources in relative positions resembling 
those expected in a plane installation. A reduction 
of sensitivity of about 8 db was required to suppress 
the local signal on the indicator lamp when two 
sources were used, 25 feet to one side of, and about 
5 feet behind, the receiver. It was hoped that use 
of shields around the receiver to limit the field of 
view and around the sources to keep radiation out 
of the field for a distance of several hundred yards 
in front of the plane would eliminate this difficulty. 

Positional Indication. At the request of BuAer, 
a receiver optical system was designed and con¬ 
structed under Section 16.5 by Contract OEMsr- 
1219 at the University of Rochester, which sent 
radiation to one of four TF cells according to the. 
position of the source—above or below or to right 
or left—of the axis of view. This system had an 
//0.56 objective made of Plexiglas elements and a 
specially designed mount of four optical bars to 
split the field into quadrants and to conduct the 
light to the four cells. The response in each quad¬ 
rant extended out to about 12 degrees from the axis. 

However, the best sensitivity with this device was 
14 db below that of the one-cell (type A) receiver 
described above, and was only 5 db above the re¬ 
sponse of a bare type B TF cell without any optical 
system. It was concluded that the value of the 
quadrant indication was not worth this sacrifice in 
sensitivity. 

Test Beacon. A test beacon constructed for check- 





192 


NIR RECOGNITION AND CODE COMMUNICATION SYSTEMS 


ing the operation of the PR receiver in the field has 
already been described in Section 4.1.3. 

Operational Tests 

No operational tests have been recorded except 
those mentioned previously under “Backscatter.” 

Present Status 

Three preliminary models are awaiting opera¬ 
tional flight tests at Patuxent River Naval Air 
Station. 

Recommendations 

The operational tests, when carried out, will prob¬ 
ably lead to some recommendations on the present 
units. 

In addition, it would seem important to use such 
equipment as this in investigating the aircraft 
scanning problem and the problem of electrical 
alignment with the guns, as mentioned previously 
under “Evaluation.” In particular, the use of a 
higher code frequency, perhaps up to 3,000 cycles, 
should be studied (with sources such as the cesium 
lamp with suitable TF-cell or PbS-cell amplifiers) 
in the interest of increasing feasible scanning speeds. 

54 RETRODIRECTIVE TARGET 
LOCATOR [RTL] 

Description and Performance 

Course of Development. At the request of BuAer, 
a project was inaugurated in September 1943 by 
the University of Michigan under Contract NDCrc- 
185 to construct an airborne transceiver for sending 
out a visible or NIR beam and detecting its reflec¬ 
tion from life rafts equipped with Mt. Wilson pre¬ 
cision glass retrodirective triple mirrors. 10 The 
orders of magnitude of attainable ranges, both from 
air-to-sea and from land-to-land stations, under 
various conditions were to be determined with a 
simple working model for the purpose of guiding 
further development plans. Over-water tests of the 
model were carried out at the Patuxent River Naval 
Air Station in December 1943 with the cooperation 
of BuAer. 

General Design. The RTL system requires a 
transceiver at one station and one or more triple 
mirrors at the other. The general design of the RTL 
transceiver is shown in Figures 17 and 18. Because 
of the highly accurate retrodirective property of the 


Mt. Wilson triple mirrors—the deviation of the re¬ 
flected beam being held by the manufacturing tol¬ 
erances to less than 5 seconds of arc—it is essential 
that source and detector be as closely adjacent as 
possible. To achieve this a small source is mounted 
directly in front of the central area of a large re¬ 
ceiver aperture. 

The source consists of a 300-watt, 30-volt, T-10 
projection lamp with C-13 filament (see Section 
1.2.1), backed by a spherical mirror of 3-inch aper¬ 
ture placed at the focus of a Fresnel-type optical 
signal lens of 5%-inch diameter and 3V2-i n ch focal 
length. This arrangement provides a beam with hpi 
width about 4 degrees and maximum intensity about 
125,000 candlepower. Use of an NIR filter equiva¬ 
lent to Polaroid XR7X25 reduces the intensity to 
about 40,000 effective holocandlepower. A beam 
spreader may be inserted to increase the hpi width 
to 10 degrees. 

The light is modulated at 90 cycles by passage 
through two opposing 90-degree slots cut in a cylin¬ 
drical tube or chopper rotating about the lamp. The 
chopper is driven by a 24-volt, 2,700-rpm, d-c, 
governor-controlled motor. The lamp and motor are 
in a vertical chimney ventilated by a blower on the 
motor shaft. The receiver is carefully shielded from 
the source by light-tight joints except for a small 
out-of-phase controlled leak used as compensation 
for atmospheric backscatter (see Section 4.1.3). 
Backscatter from local objects is cut down by plac¬ 
ing an extension tube out in front of the Fresnel 
lens, as shown in Figure 18. The receiver consists of 
a type A TF cell mounted axially at the focus of 
a 12-inch diameter, 2 1 /o-inch focal length Alzak 
aluminum reflector, with about a 15-degree hpr 
angle of view. The preamplifier, amplifier and tuned 
filter circuit are of the design used in the type D 
development in late 1943 (see “Preamplifier” under 
Section 5.2); the response peaks at 90 cycles with 
a 5-cycle bandwidth. 

An output transformer may be used to operate 
either headphones or a neon indicator lamp, pref¬ 
erably the latter. The transceiver is mounted in 
gimbals permitting an angle of swing of 70 degrees. 
The total weight together with a small control and 
indicator panel is about 115 pounds in the labora¬ 
tory model. A telescopic aiming sight is provided. 

The retrodirective triple-mirror reflectors, shown 
in Figure 19, were furnished by Section 16.5, NDRC. 
Each consists of a tetrahedral glass prism with 


iMtjgrenrTKD 





RETRODIRECTIVE TARGET LOCATOR [RTL] 


193 



Figure 17. Design of RTL transceiver. 


three internally reflecting silvered faces which form 
the corner of a cube. An incident beam is reflected 
three times in such a corner to return accurately 
along its initial path. This effect is commonly used 



Figure 18. RTL plane installation, exterior. 


in highway markers and signs to reflect lights from 
auto headlights back to the driver. The effective 
entrance angle of these mirrors is about 90 degrees 
wide; six to eight reflectors will give complete 
coverage over a hemisphere, as for use on a life 
raft. The effective diameter of the active face of 
one mirror is about 40 mm, and the diffraction width 


of the reflected beam from a face of this area is 
comparable to the allowed deviation of 5 seconds. 
This deviation corresponds to 1.5 inches per mile, 
so that the reflected beam from the transmitter 
used here falls completely within the receiver at 
distances up to at least two miles. Each mirror 
weighs about 0.25 pound. Several mirrors may be 
mounted adjacently to give increased intensity. 

The largest ranges are obtained when the beam 
is interrupted at the mirrors by a rotating sector 
disk, as shown in Figure 19. The motor actually 



Figure 19. RTL reflector modulation unit. 

used with the three-bladed sector disk shown was a 
24-volt, 1,800-rpm, governor-controlled, d-c motor. 
Security. The filter used for the NIR studies was 


































































































194 


NIR RECOGNITION AND CODE COMMUNICATION SYSTEMS 


equivalent to Polaroid XR7X25. A rough computa¬ 
tion indicates that the maximum NVR is of the 
order of 200 yards. The filtered beam intensity is 
about 30,000 holocandlepower, about 10 times as 
strong as type E and 100 times type D, with the 
result that the maximum ACW range of observa¬ 
tion of the source by an infrared electron telescope 
such as the C 3 or C 4 must be quite large, of the order 
of 10 miles for the 4-degree beam. 

The RTL system has some unusual security fea¬ 
tures, because the reflectors can be detected over 
a very wide angle, but only from an observing point 
within a few inches of the transmitter beam. They 
are inconspicuous in the daytime, and enemy cap¬ 
ture would reveal a minimum of information as to 
their mode of use. 

Range. With the array of three reflectors shown 
in Figure 19 the ACW night ranges of the working 
model appear to be about as follows between ground 
or ship stations: 


Source Modulated 
(with compensation Reflectors 
Beam width for backscatter) Modulated 


4° no filter 
Filter 

10° no filter 
Filter 


2.5 miles 

2.0 miles 

1.5 miles 


4.5 miles 

3.5 miles 

3.5 miles 

2.5 miles 


Due to various operating difficulties, some of which 
might be present in actual use, no range from an 
airborne transmitter of 10 degrees hpi width to a 
ground reflector was obtained greater than about 
1 mile. Daytime ground ranges were less than 0.5 
mile. 

Evaluation. The working model was designed to 
give only order-of-magnitude range determinations 
to guide further plans and not absolute limit ranges 
for this type of equipment. How much the improve¬ 
ments made in the type D receiver sensitivity in 
the past two years would improve the performance 
of the system is unknown. 

All reflection systems such as this system or 
IRRAD (infrared radar, Chapter 6) involve a dou¬ 
ble attenuation of the light beam since it traverses 
the path twice; also the inverse square law (Section 
4.1.3) generally, though not always, operates over 
both paths, giving an inverse fourth-pow r er law with 
distance, in addition to the attenuation. As a result, 
while a one-way system like type E (Section 4.4.2) 
has a loss of signal near its operating limit at the 
rate of about 7 db per mile in average clear weather, 


the RTL system has a rate of loss near its operating 
limit of about 20 db per mile. Even with consider¬ 
able improvement in performance, it therefore seems 
unlikely that the ACW range of the RTL system, 
with the modulated source, would increase much 
beyond 4 miles or, with the modulated reflectors, 
much beyond 5 miles. It should be possible to in¬ 
crease the air-to-ground (or water) range by use 
of a slower plane (that in the tests had a cruising 
speed of about 180 mph) with provision of a large 
field of view and certain automatic helps for the 
operator. Daytime ranges up to 1 mile could prob¬ 
ably be obtained with slight revisions in design. 

It would therefore appear that a system of this 
kind could be made to serve fairly well the purpose 
for which it was intended. 

As already noted in Section 5.1.1, the Navy spe¬ 
cifically did not want telescope detection of the 
reflected beam because such detection would require 
an extra crew man on the plane for constant search¬ 
ing. However, some observations made during the 
development throw light on the relative usefulness 
of a telescopic RTL. It was noted in the tests that 
at threshold range for the present system, when 
no filter is used, the reflectors are clearly visible to 
an observer whose eye is several inches away from 
the most intense part of the beam. This suggests 
that a small ordinary telescope substituted for the 
receiver might give a considerably greater range 
of detection, and would give a much more direct, 
certain, and accurately located observation of the 
reflector than the mere observation of the RTL neon 
indicator lamp could ever give; and source or re¬ 
flector modulation would be unnecessary. Judging 
from image tube versus receiver sensitivities ob¬ 
served with other systems, an infrared electron tele¬ 
scope used with a filtered RTL beam would have a 
range somewhat less than that with the present re¬ 
ceiver; however, it would still possess the other 
telescope advantages just noted. 0 

Transmitter Details 

Compensating Device. The details of the device 
which compensates for backscatter are rather inter¬ 
esting. A small hollow tube projects radially out to 
one side of the lamp as shown in Figure 17. In this 
tube is placed a Lucite rod with a roughened conical 
taper on the external end. This rod can be pushed 

c Infrared retrodirective telescope systems are described 
in Division 16 Summary Technical Report, Volume 4. 






RETRODIRECTIVE TARGET LOCATOR [RTL] 


195 


in or pulled out of the tube to adjust the amount 
of light escaping from its tip to the receiver mirror. 
The hollow tube may be moved laterally to adjust 
the phase of the compensating light so it is exactly 
90 per cent out of phase with the transmitter beam. 
The amount of compensation must be changed with 
changing source location and weather conditions. 
During an operation the amount of compensation 
is adjusted so that the signal is a minimum when 
none of the triple mirrors is in the field of view. 
Frequent adjustment of the compensation is re¬ 
quired on account of changing atmospheric condi¬ 
tions and it should be made automatic in any 
equipment designed for actual field use. 

Operational Tests 

Tests over Land. Many night tests were carried 
out in 1943 over land from a transceiver in a 
seventh-floor room in the University Hospital in 
Ann Arbor, Michigan, to a series of reflector sta¬ 
tions from 0.2 to 5.3 miles away down the Huron 
River Valley. If the signal from a given station 
were above threshold, it could be reduced to thresh¬ 
old by reduction of the source candlepower with a 
calibrated rheostat. The amount of reduction re¬ 
quired was an indication of the additional range to 
be expected with full source {5ower. The resultant 
threshold ranges, corrected to ACW and averaged, 
are roughly as given above under “Range,” for re¬ 
ceiver modulation. For source modulation, all of 
the additional range indicated by this procedure 
cannot be used because the local backscatter in¬ 
creases as the source power is increased to its full 
value; even with compensation, the noise level rises. 
Estimated suitable corrections to take account of 
this situation were included in computing the range 
values previously given for source modulation. 

Some uncertainty in these measurements was in¬ 
troduced by train smoke from a nearby railroad. 
It was noted that operation through snow or rain 
was impossible because of the strong noise pro¬ 
duced by pseudomodulation of the backscattered 
radiation by the falling drops or flakes. 

Tests at Patuxent River Naval Air Station. With 
the cooperation of BuAer, the transceiver was 
mounted on a modified port hatch cover of a B-26 
type medium bomber and used in day and night 
tests to reflectors on the ground from altitudes of 
1,000 to 5,000 feet. These tests were carried out from 
December 29, 1943, to January 1, 1944, inclusive. 


The transceiver had a vertical traverse of about 
70 degrees which limited the time for identifica¬ 
tion of the target by the operator and hand aiming 
of the equipment to little more than 5 seconds at the 
plane speeds and flight altitudes used. The field of 
view of the operator was very limited and was dif¬ 
ferent from that of the pilot. The result was that 
very few runs made successful contact. Night con¬ 
tacts were obtained only at ranges under 6,000 feet. 
No successful contacts at all were obtained in the 
daytime. 

The night ranges are less than the values over 
land previously noted, partly because of the experi¬ 
mental difficulties and partly because of overcast 
cloud layers near the working altitudes, which would 
not have interfered with operation between land 
stations. The apparatus appeared to function almost 
as well in the plane as on the ground. Daytime iden¬ 
tification was impossible because of high and irreg¬ 
ular background illumination (about 1,000 foot- 
candles) from the snow-covered landscape. One 
daytime test was made at 1,000 feet altitude over 
a reflector mounted on a small boat on Chesapeake 
Bay. This was unsuccessful. 

Subsequently, a theoretical study was made of 
optimum relations between scanning angle and rate, 
plane velocity and altitude, and beam width for 
most efficient search from a plane (as for survivors 
at sea) with an instrument like the RTL. 10 

Tests over Water. Additional tests were made 
January 6, 1944, from the RTL unit on a low bluff 
to retrodirective mirrors on a small boat in Chesa¬ 
peake Bay. The night was clear and the ranges 
obtained agree very well with those given previously 
under “Range” for measurements over land. 

Present Status 

Nothing further was done with the RTL follow¬ 
ing these tests, because at that time the Navy felt 
that radar methods offered greater promise and 
precluded the addition of other equipment on planes 
already heavily loaded. 

Recommendations 

The principles of retrodirective location with tri¬ 
ple mirrors should be extremely valuable for both 
military and civilian purposes. The RTL equipment 
demonstrated the feasibility of one possible mode 
of application. 







196 


NIR RECOGNITION AND CODE COMMUNICATION SYSTEMS 


5.5 JAPANESE INFRARED DETECTION 
[JAPIR] SYSTEM 

Description and Performance 

Course of Development. In 1944 Navy observers 
reported that the Japanese air forces seemed to be 
successful in assembling during night operations 
without the use of detectable radio or visible sig¬ 
nals. This suggested that they might be using infra¬ 
red recognition or signaling devices. Project Con¬ 
trol NA-191 was therefore set up in July 1944 
under Contract NDCrc-185 at the request of 
BuAer to develop and construct four sets of equip¬ 
ment for aircraft installation to detect such infrared 
signals if they were being used. 11-13 

These units were to give automatic instrument 
panel presentation, were to be bore-sighted with the 
guns with hpr width of 6 degrees, and were to have 
minimum weight, power consumption, and aerody¬ 
namic drag. A threshold sensitivity of 0.1 mile- 
candle (about 10“ 9 lumens for a 7-inch mirror) was 
requested, but, of course, is out of the question with 
present detector cells with a suitable bandwidth in 
any feasible size of reflector (see Sections 4.1.3 and 
4.4.2). In absolute darkness, a sensitivity of about 
5 modulated mile-holocandles (see the Appendix) 
was actually obtained with the apparatus built. 
The incident flux on the cell with this signal is less 



Figure 20. JAPIR nacelle, mounted on wing, covers 
removed. 


than y 1 o of the incident NIR flux from a clear 
moonless night sky. Because of this light from the 
night sky, under actual operating conditions the 
threshold may be increased to 50 mile-holocandles 
or more. 

One unit was mounted in a nacelle on a Grumman 


Hellcat, as shown in Figure 20, and flight-tested at 
the Patuxent River Naval Air Station. Six complete 
units were constructed and furnished to BuAer in 
1945. Two of these were intended for spares. 



Figure 21. JAPIR reflector-motor unit, with filter. 

General Design. A JAPIR unit consists of a re¬ 
ceiving head, amplifier, and indicating light. The 
head (Figure 21) contains a %x %-inch TF cell 
mounted axially at the focus of a T^-inch diameter, 
1-inch focal length Alzak aluminum mirror, giving 
an hpr angle of view of about 6 degrees. The cell 
is supported on a Lucite disk, coated with an infra¬ 
red transmitting film filter which covers the mirror. 
This filter has a short wavelength cutoff at 0.8 \i, 
which reduces the effect of moon and sky light but 
has little effect on an already filtered NIR signal. 

The cell fits inside a light chopper, which is a 
cylindrical bakelite tube concentric with the axis 
of the mirror. The tube has two opposing 90-degree 
sectors cut out of it and can be rotated by a 28-volt, 
aircraft-type, 3,600-rpm, d-c motor mounted behind 
the mirror so as to interrupt an incoming beam at 
120 cycles. The chopper provides an a-c component 
in the amplifier pass band between 60 and 500 
cycles, in case the enemy infrared source being 
sought is either unmodulated or is modulated with 
frequencies outside this pass band (as in voice 
communication). A switch on the control panel 
provides for stopping the chopper and holding it in 
the open position if it is desired to receive signals 
modulated in the pass band, or for stopping it and 
holding it in the closed position for protection of 
the cell against sunlight when the equipment is not 
in operation. 






JAPANESE INFRARED DETECTION [JAPIR] SYSTEM 




197 


A small low-voltage lamp mounted close to the 
cell (Figure 21) may be turned on with the chop¬ 
per running to check cell sensitivity and operation 
of the equipment in the field. The preamplifier, 
mounted with the receiver head and chopper motor 
in the nacelle shown in Figure 20, consists of a 
cathode follower and one stage of amplification. A 
shielded cable connects this to the amplifier control 
panel in the cockpit. The latter has a sensitivity 
control and a high-gain pentode stage and finally 
feeds a small neon glow lamp on the airplane’s 
instrument panel. 

The filament current is taken from the plane’s 
28-volt d-c supply; the plate and TF-cell voltages 
are furnished by Navy B batteries. The power 
requirement is 65 watts with the chopper motor 
running and 8 watts with it off. The weight of the 
whole equipment, including connectors and cable, 
is 25 pounds. 

Sensitivity. In absolute darkness the threshold 
sensitivity is about 5 modulated mile-holocandles. 
Under filtered unmodulated night sky light, the 
sensitivity may be less (threshold greater) by a 
factor of 5 or 10. If the signal and the night sky 
light must both be modulated by the local chopper, 
the signal sensitivity is still less. 

The order of magnitude of the maximum possible 
range of detection of an enemy NIR aircraft source 
may be estimated from the candlepower of strong 
American NIR aircraft transmitters. The 1.400-hcp 
P-G transmitter (Section 4.4.3), for example, if 
modulated below 600 cycles, should be detectable in 
the center of the beam by JAPIR equipment to 
about 30,000 yards vacuum range, 10,000 yards 
ACW range. The Patuxent River plane-to-plane de¬ 
tecting tests, to be described later, gave a range of 
detection on moonless nights of 3,000 yards when 
the source was a mechanically coded 240-watt air¬ 
plane headlamp enclosed in a red glass H globe. 
This range was considered satisfactory. Moonlight 
seriously interfered with the operation of the equip¬ 
ment. Actual detection ranges under combat or 
nightfighter conditions evidently will depend 
strongly on sky conditions, beam widths and direc¬ 
tions and on the circumstances of the encounter. 

Evaluation. The JAPIR system fills all the ini¬ 
tially requested military characteristics except sen¬ 
sitivity, and it has as high a sensitivity as is feasible 
at present or useful with the specified beam width 
under night sky illumination. It is not especially 
suited for detecting voice systems, but it may be 


that the system represents a suitable compromise 
between maximum chopper frequencies, common 
voice frequencies, and narrow bandwidth. 

Receiver Details 

Cell Arrangement. To avoid large spurious signals 
from capacitance effects when the chopping shutter 
is rotating, the cell is surrounded by a tight-fitting 
metal shield. The chopping shutter is made of bake- 
lite for the same reason. The metal shield is solid 
except for the areas through which the radiation 
passes, which are covered by wire mesh. In the first 
model the NIR filter was placed directly around the 
shield, but sunlight focused by the mirror burned the 
bakelite chopper at some time when the outside fab¬ 
ric cover of the unit was left off in the daytime. In 
the final models the filter is therefore over the front 
of the mirror to reduce the intensity of sunlight 
which may come in accidentally in this way. 

Chopper Orientation. When the chopper motor is 
turned off, after a few seconds’ time delay to allow 
it to stop rotating, a solenoid-ratchet arrangement 
pulls the chopper shutter slowly around. The sole¬ 
noid current passes through a slip ring on the motor 
shaft, until an insulated portion of this ring comes 
under the brush contact. The shutter then stops in 
the open or closed position, depending on which of 
two brushes the current passes through; the brushes 
are selected by a switch at the control panel. 

Preamplifier. The mounting of any low-level 
audio-frequency vacuum-tube amplifier in an air¬ 
plane is a difficult problem because of vibration. In 
particular the cathode-follower stage next to the 
cell does not amplify the signal, but does amplify 
the microphonics. To prevent increases in noise from 
this cause the preamplifier was placed on soft rub¬ 
ber mountings to give the system a natural fre¬ 
quency below 10 cycles. The low vibration frequen¬ 
cies are then cut out by the amplifier pass band. 
Amplitude-limiting stops on the mountings prevent 
excessive strains from landing shocks. 

Since a single battery provides all the plate sup¬ 
ply and cell voltages, decoupling filters are required 
in the circuits to prevent regeneration. The 10-meg¬ 
ohm load resistor for the TF cell is a commercial 
composition resistor chosen for low noise. In order 
to keep the operating controls at a minimum this 
resistor and the cell voltage were not made variable. 
A voltage divider is used in the high-gain triode 
stage following the cathode follower, to adjust the 
overall amplification to the desired value. A low- 




198 


NIR RECOGNITION AND CODE COMMUNICATION SYSTEMS 


impedance stage follows the triode to permit trans¬ 
mission over the shielded cable to the amplifier. 

Amplifier. The neon glow lamp could be operated 
directly from the plate of the high-gain pentode 
stage, but a low-impedance stage is inserted between 
them to reduce insulation difficulties when the 
equipment is used in the moist tropics. Operation 
is on the linear part of the amplifier characteristic 
curve for signal voltages near threshold. Stronger 
signals may produce distortion but this is imma¬ 
terial with the neon-lamp form of presentation. 

Test Lamp. The low-voltage test lamp in the 
receiver head is preset by adjustment of a series 
resistor, so that with the chopper running it gives a 
signal on the cell corresponding to a receiver illu¬ 
mination of about 40 sea mile-holocandles. The neon 
indicator lamp should then be just extinguished at 
a certain designated setting of the sensitivity dial 
if the equipment is functioning properly. 

Control Panel and Operation. The amplifier con¬ 
trol panel was made as simple and compact as pos¬ 
sible. Space was found for it and the battery box on 
the cockpit floor of the Grumman Hellcat used in 
tests. The panel contains the sensitivity control and 
four switches: (1) the master switch, (2) the chop¬ 
per motor switch, (3) the shutter open-close switch, 
and (4) the test lamp switch. 

Operation and minor maintenance is described in 
the instruction book for operators. 12 When a JAPIR 
search is to begin, the master switch is turned on 
and the shutter opened. The sensitivity control is 
turned to the point where the neon lamp is just ex¬ 
tinguished (with no lights in the field of view) ex¬ 
cept perhaps for occasional flashes. The presence of 
an NIR modulated source in the field of view then 
produces a steady glow in the lamp. No further at¬ 
tention is necessary unless the presence of an un¬ 
modulated NIR source on a particular target is in 
question. The chopper motor must then be turned 
on and the sensitivity control perhaps reset before 
aiming at the target. If the target source is coded, 
the indicator lamp will follow the code. 

Operational Tests 

Early ground tests were made on a clear, moon¬ 
less night, at a location away from city lights; the 
light from the stars was found to give a strong 
signal. 

Later, plane-to-plane tests 13 were made at the 
Patuxent River Naval Air Station on the nights of 


December 18 and 26, 1944, and January 11, 1945, 
at altitudes of 4,000 to 6,000 feet. The F6F-3N test 
plane made range runs on a TBM target plane car¬ 
rying a 240-watt mechanically coded airplane head¬ 
lamp bulb enclosed in a red glass H globe on the 
under side of the tail. Ranges were determined by 
radar. On the first night, with visibility 6 miles, the 
maximum range received was 3,000 yards; on the 
second night, with visibility 10 miles, moonlight 
interfered and no target indication was observed; 
on the third, with visibility 8 miles, the range was 
again 3,000 yards. This range is equivalent to a re¬ 
ceiver sensitivity of about 200 mile-holocandles. 
There was no interference to or from the test plane’s 
other electronic equipment, and engine vibration 
did not affect sensitivity. As a result of the tests, 
several minor changes in design were made. 

Present Status 

The six complete JAPIR units requested by 
BuAer were delivered under Contract NDCrc-185 
in August 1945, and were mounted on chassis built 
by Patuxent Naval Air Station, preparatory to field 
tests in the Pacific Theater at the time the war 
ended. 

Recommendations 

The JAPIR system has characteristics as nearly 
like those originally requested as is feasible. 

If there is military need, this system might be 
modified for ship or land use and further adapted 
for monitoring enemy NIR voice communications. 
It could be adapted for reception of the IIR by 
simple interchange of cells. 


56 SUMMARY OF RECOMMENDATIONS 

If these studies are to be continued, some particu¬ 
lar and general recommendations can be made. The 
particular ones concern continuation of the work in 
progress at the termination of the war. 


System 
Type D 


Plane recognition 

Retrodirective 

locator 

JAPIR 


Recommendation 

System is satisfactory and first models 
are in field use, but several improve¬ 
ments are possible, and its interaction 
and possible amalgamation with voice 
systems should be considered 

Further tests 

None; other possible systems may be 
superior 

None; first models in field use 



SUMMARY OF RECOMMENDATIONS 


/ 


199 


Many of the general recommendations in Section 
4.9 are applicable here. 

Because of the strong and troublesome mutual 
interference between the systems here described— 
the voice communication systems of Chapter 4 (see 
Section 4.9 and “Recommendations” under Section 
5.2) and other infrared beacon, blinker, and heat 
detection systems—it would be highly desirable to 
have the future development of all types of NIR, 
HR, and FIR systems for all branches of the Serv¬ 
ice placed under a single directing agency. The 
function of the agency would be to plan and or¬ 
ganize the various projects so that mutual inter¬ 
ference and mutual backscatter w r ould be kept at a 
minimum and so that the possibilities of each appa¬ 
ratus can be developed to its inherent limit without 
hindrance from other independent (and possibly 
unknown) developments. The need in this field is 
now coming to be like the necessity for frequency 
allocation in the wildcat days of radio broadcasting 


during the 1920’s. Close liaison between projects 
and branches of the Services is not enough, as is 
abundantly testified by the parallel and interfering 
developments of type D, type E, and searchlight 
systems for ships, and of the separate plane-to- 
plane voice, plane-to-plane recognition, and non- 
NDRC plane-to-plane gunsight warning sys¬ 
tems. 

Military urgency has been a reasonable excuse 
for this procedure during the war, considering that 
these systems were only in the research stage and 
that few of them actually saw any field use, but the 
validity of this excuse has now disappeared and a 
conference should be called immediately among the 
branches of the Services interested in further infra¬ 
red developments with the object of clearing the 
situation up at once. Neglect of this can lead to 
unpleasant and possibly tragic consequences when 
the field applications of the infrared systems begin 
to be numerous. 




Chapter 6 


NEAR INFRARED SYSTEMS FOR DETECTING 
RANGE AND DIRECTION [IRRAD] 


By Winston L. Hole 


INTRODUCTION 

T he development of infrared range and direction 
[IRRAD] equipment was initiated by Western 
Electric Company Contract OEMsr-766 as a part 
of an NDRC Section 16.5 program for developing 
methods of night surveying by infrared. Other as¬ 
pects of this program are described in STR of Divi¬ 
sion 16, Volume 4. Since IRRAD does not employ 
an image-forming detector and can be used for mili¬ 
tary purposes other than night surveying, the further 
development of systems of this type was subse¬ 
quently transferred to NDRC Section 16.4. The 
IRRAD models described in this chapter were de¬ 
veloped by Western Electric Company Contract 
OEMsr-1267 and University of Michigan Contract 
NDCrc-185 as Army Project CE-22 and Navy 
Project NR-103. 

Two complete IRRAD models were constructed 
by the Western Electric Company for experimental 
use and field trial, one for the Army and one for 
the Navy. Both were designed for the detection and 
ranging of high-precision retrodirective reflectors 
(triple mirrors) and differ principally in the type 
of power supply from which they are operated. The 
Army model was designed for battery operation, 
light weight, and ready divisibility into four man- 
borne packages. The Navy model was originally 
closely analogous except in having an a-c power 
supply unit, but was subsequently modified to per¬ 
mit the mounting of the essential components on a 
gyrostabilized platform. It thus became the scan¬ 
ning head of a shipboard plan-position-indicating 
system for detecting triple-mirror targets mounted 
on other ships. 

Only one experimental model of laboratory type 
was constructed under NDCrc-185. It was designed 
for the detection and ranging of the ship proper as 
a diffusely reflecting target, without the addition 
of highly efficient reflectors of the retrodirective 
type. 


The differences between the two types of target 
called for some basic differences in design. The gen¬ 
eral description and field test results for the dif¬ 
ferent types developed for these purposes by the 
two contracts will therefore be given separately in 
the sections which follow. 

611 Principles of Operation 

The basic principle of IRRAD is identical with 
that of radar. An infrared pulse having a dura¬ 
tion of the order of 1 psec is emitted by a flash 
lamp especially designed for this purpose (Section 
1.3.1, “Type 300 Microflash Lamp”). If a target is 
within the range and field of view of the equipment, 
a small fraction of the energy in this pulse is re¬ 
flected back to the receiver. Here it is detected and 
amplified by an electron multiplier tube having an 
infrared-sensitive photoemissive cathode (see Chap¬ 
ter 3) and finally is presented as a signal on the 
screen of a cathode-ray tube. The range of the tar¬ 
get is determined from the time delay between the 
emission of the pulse and its reception after reflec¬ 
tion. A combination of several factors, namely, the 
short duration of the infrared pulse, the absence 
of pickup between the transmitter and the receiver, 
and the elimination of time delay associated with 
“transmit-receive” circuits in conventional radar 
sets, permits accurate range measurements to be 
made on targets at distances down to a few hundred 
feet. Moreover, a high degree of secrecy is main¬ 
tained because of the small angles to which 
the transmitted IRRAD beam is confined. Also, 
because the beam can be rendered completely in¬ 
visible by means of infrared transmitting filters, 
special infrared detectors not widely used in mili¬ 
tary field operations are required to detect it. Even 
with the necessary infrared apparatus the beam can 
not be detected unless the receiver is oriented in the 
proper direction during the short period required 
for the IRRAD equipment to scan a given zone. 


200 


1RRAD FOR RETRODIRECTIVE REFLECTOR TARGETS 


201 


62 IRRAD FOR RETRODIRECTIVE 
REFLECTOR TARGETS 

The first IRRAD equipment 1 was constructed 
under Contract OEMsr-766 as a laboratory experi¬ 
mental model to explore the possibilities of obtain¬ 
ing range data with an infrared system utilizing 
radar pulse techniques. The practicability of the 
method was demonstrated to representatives of 
NDRC, the Army, and the Navy at the Bell Tele¬ 
phone Laboratories in New York City on November 
18, 1943. A single target located 774 yards from 
the equipment was repeatedly ranged with a dis¬ 
tance error usually less than ±5 yards and a direc¬ 
tional error in either azimuth or elevation usually 
not greater than 0.25 degree. 

The promising results of these preliminary tests 
led to requests by the Armed Services for additional 
developmental work. Two improved models 2 ’ 3 were 
therefore constructed under Contract OEMsr-1267, 
one for the Army and one for the Navy. These 
models, which are similar in most respects, are de¬ 
scribed in the following sections. 

6,2,1 General Description of Equipment 

Figure 1 is a photograph of the complete Navy 
model IRRAD. It consists of a main unit mounted 
on a tripod and a power supply unit which can be 
placed at any convenient location near by. 

Additional developmental work was subsequently 
done on a revised Navy model for use with a plan- 
position type of indication. For this model a spe¬ 
cial scanning head, containing only the optical 
elements of the transmitter and receiver, was de¬ 
signed for mounting on a gyrostabilized platform. 
The Army model closely resembles the Navy model 
shown in Figure 1 except in the details of the power 
supply unit, and also in having a special tripod 
mounting head and cradle provided with accurately 
calibrated circles to facilitate high-precision meas¬ 
urements of the azimuth and elevation of the tar¬ 
gets. 

The interior of the main unit is shown in Figure 
2, and a functional diagram in Figure 3. However, 
the optical elements shown in Figure 3 are those 
used in the preliminary laboratory model and were 
superseded in the final models by those shown in 
Figure 4. 

Referring to Figure 3, the high-voltage power 
supply energizes the 0.1-pf condenser mounted in the 


lamp shield. When the voltage across the condenser 
reaches the firing 'potential of the lamp, the con¬ 
denser discharges through the lamp and a radiation 
pulse is emitted. The electric synchronizing pulse 
from the lamp firing circuit triggers the range unit, 
which delivers an accurately delayed output pulse 
at a round-trip time interval corresponding to the 
range in yards marked on the dial of the range unit. 



Figure 1. Complete IRRAD equipment, Navy model. 

The synchronizing pulse also triggers a special high¬ 
speed sweep circuit which deflects the cathode-ray 
beam horizontally. Combined with the sweep cir¬ 
cuit is a brightening-voltage generator which inten¬ 
sifies the cathode-ray beam during the horizontal 
sweep and makes the trace readily visible. The 
radiation pulse reflected from the target is detected 
and amplified by the photomultiplier tube and, after 
additional amplification by a thermionic tube cir¬ 
cuit, the electric impulse is applied to the vertical 
deflection plates of the cathode-ray tube. When the 









202 


IRRAD SYSTEMS 


range unit has been adjusted so that its output pulse 
coincides with the target-echo pulse on the screen 
of the cathode-ray tube, the range of the target is 
read directly from the dial of the range unit, and 
the azimuth and elevation are read from the circles 
provided for this purpose. Final “sighting” adjust¬ 
ments in each plane are made by means of slow- 
motion controls, using the amplitude of the target 
pulse as the indicator, after the slits in the receiver 
optical system (items 12 and 13 in Figure 4) have 
been narrowed sufficiently to yield the desired de¬ 
gree of accuracy. 

The dimensions of the cabinet of the main unit 
seen in Figures 1 and 2 are 24x24x8.5 inches, and 


the unit weighs about 47 pounds including covers. 
Layout drawings have been prepared for reducing 
the size of the unit to 19x19x8.5 inches with an esti¬ 
mated weight of less than 30 pounds. The power 
supply for the Army unit operates from a 12-volt 
storage battery and weighs approximately 28 
pounds. The current drawn by this power supply 
when the lamp is not flashing is about 5 amperes and 
the average total current with the lamp flashing 55 
times per second is about 11 amperes; the power 
actually delivered to the main unit divided by the 
power drawn from the battery indicates an overall 
efficiency of about 70 per cent. The power supply 
unit for the Navy model is operated from 115-volt, 


19 

11 



Figure 2. Interior of main unit, covers removed. (Key to numbers is given in Figure 4.) 




















IRRAD FOR RETRODIRECTIVE REFLECTOR TARGETS 


203 



60-cycle alternating current. The average total 
power consumed with the lamp flashing 60 times 
per second is approximately 175 watts. If desired 
for Army use also, this same power supply unit 
could be operated, for example, from an a-c dyna- 
motor driven from the battery of a truck or jeep. 
The a-c power supply weighs approximately 55 
pounds. 

It was originally requested in Army Project 
CE-22 that the total weight of the equipment should 
not exceed 50 pounds and should be capable of 
being broken into two loads, neither of which should 
weigh more than 35 pounds. Although it was im¬ 
possible to adhere rigidly to these limitations on 
weight, the Army model was designed to be subdi¬ 
vided for portability into four manborne packages, 
no one of which would necessarily exceed the 35- 


pound limitation in a final model. These packages 
include a small 12-volt storage battery and a tripod 
mount with transit head, in addition to the main 
unit and the power supply mentioned above. Al¬ 
though the total weight of the equipment demon¬ 
strated at the Army Engineer Board, Fort Belvoir, 
Virginia, was 148 pounds, the weight of certain com¬ 
ponents was subsequently reduced and a reduction 
in the "weight of certain other components is known 
to be feasible. 

Components of the Main Unit 

Transmitting Optical System 

The transmitting optical system shown in Figure 
4 consists of a short-gap, double-ended microflash 
lamp (Section 1.3.1, “The Type 300 Microflash 


"ll 1 H P T U I Vii I I'J ♦ 































































































204 


IRRAD SYSTEMS 


Lamp”), a condensing lens system, an infrared 
transmitting filter, a right-angle prism, and a nar¬ 
row fixed slit located in the focal plane of the pro¬ 
jection lens system. An elliptical first-surface plane 
mirror mounted at a 45-degree angle on the axis 
of the receiver optical system reflects the transmit¬ 
ter beam in a direction parallel to the axis of the 
receiver. With the components located in this man¬ 
ner the accurately coaxial alignment of the trans¬ 
mitter and receiver optical systems needed for 
effective utilization of the highly accurate retro- 
directive triple mirrors is maintained. In addition 
the area of the receiver mirror obscured by com¬ 
ponents of the transmitter is reduced to a minimum 
with a consequent increase in optical efficiency and 
operating range. A thin, clear glass window in the 
case of the unit provides weather protection for the 


aperture and is in turn protected by a metal cover 
while being transported. 

The desired infrared filter is selected by means of 
an external control which rotates a mount contain¬ 
ing space for three filters. The filters actually used 
were Wratten 87 and Polaroid XR7X26, each 
cemented between clear glass plates, and a clear 
glass dummy for use in daytime or at night if visual 
security is not required. The extent of the projected 
beam, determined by the constants of the trans¬ 
mitter slit and projection system, is 2.5 degrees in 
the vertical plane and 24 minutes in the horizontal 
plane. The vertical extent of the beam is ultimately 
limited by the length of the arc gap in the flash 
lamp, while the horizontal extent may be changed, 
within limits, by varying the width of the slit. For 
use on a rolling ship, even with a stabilized plat- 




9 


Key to Numbers 
(See also Figure 2.) 


1. Microflasli lamp 

2. Condenser lens 

3. Rotating filter mount 

4'. Projection filter selector 

5. Right-angle prism 

6. Narrow projection slit 

7. Projection lens 

8. Elliptical plane mirror 

9. Thin glass window 

10. Metal window cover 

11. Parabolic mirror 


12. Hinged narrow slit 

13. Fixed wide slit 

14. Iris diaphragm 

15. Red glass filter 

16. Slit-cathode relay lens 

17. Photomultiplier housing 

18. Optical system support 

19. Optical system support 

20. Optical system support 

21. Mirror adjustment control 

22. Alignment viewing hole 



7 


Figure 4. Optical schematic of IRRAD for retrodirective reflector targets. 





































1RRAD FOR RETROD1RECT1VE REFLECTOR TARGETS 


205 


form, a somewhat wider beam than 24 minutes may 
be required. Means are provided for both axial and 
lateral adjustments of the source, and for axial 
adjustments of the condenser lens, slit and projec¬ 
tion lens. 

All optical elements of transmitter and receiver 
are coated for minimum reflection loss at 0.9 p and 
are mounted on two rigidly connected cylindrical 
tubes and a supporting structure. 

Target. One or more of the autocollimating, 
trihedral retrodirective triple mirrors (see STR 
Division 16, Volume 4) used as targets is mounted 
on the object or at the place for which range and 
direction are desired. Two sizes, nominally rated at 
40-mm and 80-nim aperture, have been used. For 
field use they are mounted in waterproof spun-metal 
housings. In order to provide protection from chance 
visual observation the reflectors are sometimes cov¬ 
ered with Wratten 88A filter material cemented 
directly to the face of the prism under an optically 
flat cover glass approximately 10 nun thick. 

Receiving Optical System 

The receiving optical system, also shown in Fig¬ 
ure 4, consists of a second-surface glass parabolic 
reflector which forms an image at its focal point; 
two slits mounted at the focal plane; an adjustable 
iris diaphragm; a red-glass light filter; and a lens 
which projects the image formed at the slit onto the 
cathode of the photomultiplier tube. 

Item 12 in Figure 4 is a thin metal plate contain¬ 
ing a slit 0.010 inch wide. It is mounted on hinges 
so that it can be rotated out of the optical path by 
means of an external control. An adjacent fixed 
plate, item 13, contains a slit 0.040 inch wide. The 
wide slit is used during the initial search for a 
target and the narrow slit replaces it for making 
azimuthal settings after the target is located, since 
it further restricts the field of view and thus permits 
a more accurate reading. An iris diaphragm, item 
14, is located immediately beyond the slit. Like the 
slits it is expanded during the search and contracted 
by means of an external control for the final setting 
for angle of elevation. The minimum diameter of 
the opening in the iris diaphragm is 0.30 inch, which 
limits the accuracy in setting vertical angles to 
about ±5 minutes of arc. A diaphragm having a 
smaller minimum aperture would decrease the 
residual error from this cause. On the other hand it 
may be necessary to use much wider slits and to 


adopt other measures for increasing the angle of 
view in order to provide satisfactory continuous 
performance of a shipboard installation, unless a 
very highly stabilized platform is provided. 

The narrow slit subtends a vertical angle of 2.5 
degrees and a horizontal angle of 4.5 minutes, while 
the corresponding figures for the wider fixed slit are 
2.5 degrees and 18 minutes. Since the horizontal 
angular width of the transmitter beam is 24 min¬ 
utes, small errors in the axial alignment of the 
transmitting and receiving optical systems may 
exist without decreasing the amplitude of the signal 
received during search for a target. Also, since the 
circle of confusion of the parabolic mirror is only 
0.008 inch or less, the image of a distant target will 
pass through the narrow slit without loss. Repeated 
azimuthal settings on a distant target show errors 
ordinarily not greater than approximately ±2 min¬ 
utes of arc, as would be anticipated when using the 
narrow slit with its 4.5-minute horizontal field of 
view. A somewhat higher accuracy of setting might 
be obtained by using a mirror of still greater pre¬ 
cision and a somewhat narrower slit. 

The function of the red-glass filter, item 15, is to 
reduce the interference and multiplier noise which 
arise from scattered light during daytime observa¬ 
tions. 

A number of 14-stage photomultiplier tubes hav¬ 
ing infrared-sensitized cesium-surface cathodes 
were constructed for this project by the Farnsworth 
Television and Radio Corporation under Contract 
OEMsr-1094 (see Chapter 3). All the tubes used 
for the field tests reported in this chapter have end 
view cathodes. The tube is mounted on the axis of 
the receiver mirror in a cylindrical metal shield 
approximately 2.5 inches in diameter. Since it lies 
behind the plane mirror (item 8, Figure 4) of the 
transmitting optical system, it adds very little to 
the area of the receiver mirror that is necessarily 
obscured. At first it was hoped that the multiplier 
would provide sufficient gain to operate the vertical 
deflection plates of the cathode-ray tube without 
additional amplification, but this proved not to be 
the case. Many variations in constructional details 
were tried experimentally in an attempt to secure 
improved overall operating characteristics. For ex¬ 
ample, a 10-stage sideview tube appeared to offer 
considerable promise near the end of the contract 
period. 

The average amplification per stage for a 14-stage 



206 


1RRAD SYSTEMS 


tube may be as much as a factor of 3 for an applied 
d-c voltage somewhat in excess of 100 volts per 
stage. Under such conditions the total amplification 
of the tube is nearly 134 db. Voltage is supplied to 
each multiplier stage from the appropriate tap on a 
potential divider consisting of bleeder resistances in 
series and a small condenser connected in parallel 
with each resistance. This arrangement permits the 
use of high-resistance bleeders, since the bleeder 
current need be only a fraction of the maximum 
multiplier current at the peak of the short duration 
radiation pulses. Possible damage to the tube due to 
excessive steady illumination of the cathode is 
thereby prevented, while the potential distribution 
is restored to equilibrium during the relatively long 
intervals between IRRAD pulses. 

Radiation from the core of a type 300 lamp is 
narrowed by the transmitter slit and filtered before 
reaching the photomultiplier tubes. Typical ehT 
values are 50 per cent for a Wratten 87 and 10 per 
cent for a Polaroid XR7X26. For one good repre¬ 
sentative tube tested in darkness in the laboratory, 
the signal equivalent of noise (see the Appendix) at 
a pass-band width of 1 megacycle was found to 
be approximately two equivalent microhololumens. 
This figure indicates an order of magnitude for the 
lowest value of signal flux which would be detect¬ 
able with the IRRAD equipment under ideal condi¬ 
tions. When the Wratten 87 filter is used in the 
transmitter, the extreme range of visibility for an 
observer looking directly into the unit is about 
400 yards; with the Polaroid XR7X26 filter, the 
beam is completely invisible to the dark-adapted 
eye at any distance. 

Receiver Circuits 

Signal Amplifier. In order to provide accurate 
transmission of the electric pulses generated from 
the extremely short radiation pulses, a low-output 
impedance of approximately 3,300 ohms is used in 
the collector circuit of the photomultiplier tube. 
The peak signal voltage available across this im¬ 
pedance without overloading the multiplier is about 
0.5 volt. Additional amplification of some 40 db is 
required to obtain a vertical signal deflection of 
0.5 inch on the cathode-ray tube. This is provided 
by a 2-stage, wide-band signal amplifier using 
miniature pentode tubes. The heaters of the two 
tubes are connected in series for operation either 
from a small 12-volt battery or from a 12.6-volt a-c 


supply. Plate circuits of both stages contain induct¬ 
ance to extend the high-frequency response as well 
as supply filters to maintain the low-frequency gain. 
The amplifier is constructed on a small chassis 
mounted at one end of the cathode-ray tube circuit 
compartment, just below the output end of the 
photomultiplier tube. Short leads and careful shield¬ 
ing are required to avoid pickup of transients from 
the flash lamp circuit. 

A range index pulse of adjustable amplitude and 
negative polarity, introduced across a cathode re¬ 
sistor in the second stage, appears at the output 
with the same polarity as the signal pulses. The 
combination of cathode bias with negative grid bias 
places the operating point of the tube near cutoff. 
For this reason the components of the noise input 
having polarity opposite that of the signal are 
limited at the second grid. 

Cathode-Ray Tube and Control Circuits. A type 
5HP1 cathode-ray tube with a green, medium- 
persistence screen was selected for the IRRAD 
equipment. This tube has a rating of 2,200 volts 
between cathode and accelerating anode and a de¬ 
flection plate sensitivity of about 80 volts per inch. 
A sweep expansion of about 6 psec per inch makes 
it possible to display the entire calibrated range of 
4,500 yards on the 5-inch screen when the range 
dial, which in effect moves the trace horizontally 
across the screen, is set at its mid-position. Thus a 
single trace serves to indicate the presence of a 
target at any range during search. Observed target 
signals can then be accurately superimposed on the 
range index, which remains at the center of the 
screen, by adjusting only the range dial without 
further adjustment of the sweep circuit. 

A cable from the power supply unit carries 6.3- 
volt a-c leads for the tube heater, and a negative 
2,000-volt d-c lead to supply a bleeder consisting 
of a number of fixed resistors and potentiometer 
controls for adjusting the intensity and focus of the 
beam. A voltage-regulator tube connected from a 
tap on the bleeder to ground provides a negative 
bias source of about 60 volts for a number of tubes 
in the range unit and signal amplifier. The deflection 
plates are connected through high resistances to the 
regulated potential supply for the accelerating 
anode, which is 150 volts above ground. Since this 
voltage is near the balance potential of the sweep 
amplifier-inverter circuit, good focus of the beam is 
obtained throughout the trace. The grid of the 


i 






IRRAD FOR RETRODIRECTIVE REFLECTOR TARGETS 


207 


cathode-ray tube may be varied from minus 20 to 
minus 100 volts with respect to the cathode by the 
manual intensity control on the front panel of the 
main unit. The grid is also connected to a blocking 
condenser, through which is fed a positive, square- 
top pulse from the range circuit. This pulse turns 
on the beam at the start of the sweep and blanks 
it out during the retrace and the remaining interval 
between sweeps. 

Range and Sweep Circuits. All components of 
the range and sweep circuits are assembled and 
wired in a 5x7x2-inch aluminum chassis which 
weighs about 2.5 pounds complete. Miniature 6.3- 
volt heater-type tubes are used with the heaters 
wired in series-parallel for operation either from a 
12-volt battery or from a 12.6-volt a-c supply. 

The range circuits consist of a start-stop circuit, 
an RC time-delay circuit and a coincidence circuit 
for timing purposes, and a range pulse generator 
circuit to provide an index on the screen of the 
cathode-ray tube. These circuits utilize five vacuum 
tubes and include the range potentiometer mounted 
on the front panel of the main unit. Four additional 
tubes are needed for the sweep generator circuit and 
sweep amplifier-inverter circuit which generate and 
amplify the horizontal sweep voltages used on the 
cathode-ray tube. In addition, a neon voltage- 
regulator tube provides the negative bias voltage 
required by several tubes in these circuits and the 
signal amplifier. For circuit diagrams and full de¬ 
tails of circuit operation, reference is made to the 
contractor’s report. 2 A brief description of the cir¬ 
cuits and of the functions of the components is given 
in the following paragraphs. 

A double triode connected as a biased multi¬ 
vibrator is used in the start-stop circuit. Varistor 
networks on the flash-lamp housing and in the 
range unit serve to reduce the amplitude of the 
negative synchronizing pulse derived from the cur¬ 
rent through the flash lamp and to clip any positive 
return or oscillation. The synchronizing pulse trig¬ 
gers the multivibrator circuit, which produces a 
single rectangular output pulse and then remains 
cut off until the next synchronized pulse arrives. A 
deblanking pulse of about 30 volts is taken from 
the positive rectangular wave on the first plate of 
the double triode to the grid of the cathode-ray tube. 
The second plate is coupled to the grid of a pentode 
in the RC time-delay circuit, causing a sharp cutoff 
of its plate and screen current at the beginning of 


the timing interval. During this interval the volt¬ 
ages at both plate and screen rise exponentially 
with the same initial slope toward the common 
supply voltage as an asymptote, the time constant 
of the plate circuit being adjusted for a rise to one- 
third of the asymptote in the time interval corre¬ 
sponding to the maximum calibrated range of 4,500 
yards on the range potentiometer dial. 

The plate of the pentode in the time-delay circuit 
is directly coupled to the grid of a second pentode 
in the coincidence circuit. The screens of the two 
pentodes are connected together, the second pentode 
being normally cut off until the RC timing wave 
initiates conduction. The time delay before conduc¬ 
tion starts is controlled by the setting of the range 
potentiometer which determines the cathode bias of 
the coincidence circuit pentode. 

The range pulse generator circuit contains two 
more pentodes. The output of the pentode in the 
coincidence circuit is amplified by the first pentode 
in the range pulse circuit, differentiated, and applied 
to the grid of the second pentode as a positive, 
steep-fronted exponential pulse of 20 to 30 volts 
amplitude. This positive pulse causes saturation 
current to flow to the plate and screen of the second 
pentode, and the plate drop is again differentiated 
by a circuit in parallel with the range index ampli¬ 
tude-control potentiometer to form the exceedingly 
sharp single negative range pulse which appears on 
the screen of the cathode-ray tube. 

The horizontal sweep circuits contain two triodes 
in the sweep generator circuit followed by two 
pentodes in the sweep amplifier-inverter circuit. 
The first triode controls the cathode bias of the 
second in such a way that the grid cutoff potential 
of the latter is determined by the range potentiom¬ 
eter setting. This tube is biased so that it is always 
brought into the conducting region by the RC timing 
wave a few microseconds before the range pulse 
circuit tubes in order that the range index will 
always appear on the sweep trace. A further advan¬ 
tage is that the range index is automatically cen¬ 
tered on the screen for all range potentiometer 
settings, the target signal being carried to the center 
of the tube when it is aligned with the index to 
obtain range readings. The sweep amplifier-inverter 
circuit contains two pentodes and is direct-coupled 
throughout. Negative shunt feedback is utilized to 
stabilize the gain, to provide inverter action, and to 
furnish a reasonably low output impedance into the 





208 


IRRAD SYSTEMS 


cathode-ray tube horizontal deflection plates, 
thereby minimizing pickup or distortion and re¬ 
ducing the delay in response due to stray capaci¬ 
tance. The voltage to the deflection plates is essen¬ 
tially the same as that from a push-pull amplifier, 
and both tubes are quite linear for wide excursions 
away from the balance point. 

Wire-wound resistors, silvered mica condensers 
having close tolerances and ceramicon condensers 
having a negative temperature coefficient to com¬ 
pensate for temperature drift are used in the timing 
circuits. The range potentiometer is accurately 
wound on an exponentially tapered card, so de¬ 
signed that the relation of the resistance to the 
angle setting of the slider arm matches the exponen¬ 
tial rise of voltage at the plate of the time-delay 
circuit pentode, and thus enables the range dial to 
have a linear scale. The dial scale contains 450 
divisions, the range unit calibration being adjusted 
to a scale factor of ten yards per division and 
checked against a crystal-controlled range calibrat¬ 
ing circuit during construction. Errors are removed 
by adjustment of a trimmer condenser. A “zero 
adjust” potentiometer is mounted on the panel of 
the main unit to allow the operator during field use 
to compensate for possible time delay of perhaps a 
fraction of a microsecond between the synchroniz¬ 
ing pulse, the peak of the radiation flash, and the 
time at which the range pulse generator tubes begin 
to function. 

In setting the “zero adjust” knob a target should 
be placed at a known distance, say 50 yards, the 
range dial set for this distance, and the range pulse 
adjusted to coincidence with the signal pulse on the 
oscilloscope screen. The best accuracy in range 
measurements is obtained if the pulses are brought 
to coincidence in the same manner as for the zero 
set, preferably coinciding on their steep leading 
slopes. For this reason each operator should make 
his own zero set and should check it repeatedly 
during any series of important measurements. 

62 3 D-C Power Supply 

The d-c power supply for the Army equipment 
operates from a 12-volt storage battery and con¬ 
sists of two principal parts: (1) the converter, 
which develops minus 2,000 volts, plus 300, plus 225, 
and plus 150 volts d-c, and 6.3 volts a-c; (2) the 
pulser, which energizes the flash-lamp condenser. 


Several novel features have been successfully in¬ 
corporated in the converter. The converter relay is 
a mechanical oscillator employing a new type of 
vibrating element which is balanced magnetically 
and is tuned electrically to have a frequency of 
about 110 cycles per second. In operation, it sends 
current from the battery alternately through the 
two halves of the primary of the converter trans¬ 
former, thereby setting up alternating voltages in 
the three secondaries of the transformer. Each sec¬ 
ondary consists of two windings, one of which is 
associated with each half of the primary winding. 
The vibrator armature and two fixed contacts are 
sealed in a small glass tube containing hydrogen 
under high pressure, the entire tube being enclosed 
by the relay winding. The elements are designed so 
that the contacts are continuously wetted with 
mercury from a small reservoir contained in the 
tube. The mercury insures extremely low contact 
resistance and also provides “closed transfer” be¬ 
tween the primary windings by permitting the 
armature to touch one contact for an instant before 
separating from the other as it oscillates back and 
forth. A novel magnetic structure has been incorpo¬ 
rated in the converter transformer in order to utilize 
more advantageously the closed transfer feature. 

The current drawn from the battery by the con¬ 
verter is practically constant. The secondary volt¬ 
age wave is essentially square and produces a sub¬ 
stantially steady voltage at the output of the full- 
wave vacuum-tube rectifiers used for the minus 
2,000-volt and the plus 300-volt supplies, requiring 
less filtering than a rectified sine wave. Stabilized 
potentials of 225 and 150 volts are derived from 
the plus 300-volt rectifier by means of two gas-tube 
voltage regulators in series. The 6.3-volt a-c supply 
for the cathode-ray tube filament is furnished by 
another converter-transformer secondary, properly 
insulated from ground. Power for the dial light and 
for the range unit and amplifier tube filaments is 
supplied directly from the 12-volt battery. 

A relay similar in principle to the converter relay 
is used in the pulser, together with a specially de¬ 
signed airgap transformer having a high step-up 
ratio. Alternating current from the converter drives 
the pulser at the same frequency as the converter, 
but only when d-c bias from the plus 225-volt 
supply is applied by closing the “send” switch on 
the front panel of the main unit. Energy from the 
current which flows through the primary of the 








IRRAD FOR RETRODIRECTIVE REFLECTOR TARGETS 


209 


transformer in the half-cycle during which the 
pulser relay contacts are closed is stored in the air- 
gap, since a flow of current in the secondary of the 
transformer is prevented by the polarity of a mer¬ 
cury vapor rectifier connected in series with it. 
When the pulser relay contact is broken, the polarity 
of the voltage in the secondary coil is reversed and 
energy is stored and retained in the flash lamp 
condenser by the action of the rectifier. This process 
is repeated until the voltage across the condenser 
is sufficient to flash the lamp, dissipating the energy 
stored in the condenser. Under average conditions 
two increments of charge in succession will cause 
the lamp to flash, resulting in a flashing rate which 
is one-half the frequency of the converter (or 
pulser) relay. The flashing rate may vary with a 
number of factors such as the battery voltage, the 
breakdown potential of the flash lamp being used, 
etc. For example, as the battery voltage drops, less 
energy will be transferred to the condenser per cycle 
and more charging increments will be required to 
fire the lamp, with a consequent decrease in flash¬ 
ing rate. No disadvantage results from this aside 
from a decrease in the brightness of the trace on 
the cathode-ray tube screen. 

62 4 A-C Power Supply 

The power supply for the Navy equipment oper¬ 
ates from a 115-volt, 60-cycle alternating current. 
Output of minus 2,000 volts of direct current for 
the cathode-ray tube and the photomultiplier is 
obtained from a half-wave vacuum-tube rectifier of 
conventional design. A full-wave vacuum-tube recti¬ 
fier, also of conventional design, provides plus 300 
volts for the range unit and amplifiers, while sta¬ 
bilized voltages of plus 225 and plus 150 volts for 
these units are derived from the 300-volt rectifier 
output by two gas-tube voltage regulators. A 6.3- 
volt supply for the filament of the cathode-ray tube 
is furnished by a winding on the 2,000-volt supply 
transformer. The 12.6-volt supply for the dial light, 
the flash lamp control relay, and the filaments of 
tubes in the range unit and amplifiers was obtained, 
as a matter of expediency, by connecting two 6.3- 
volt transformer windings in series. 

The output of minus approximately 4,000 volts 
d-c for exciting the flash lamp is furnished by 
another half-wave vacuum-tube rectifier. The 
transformer used for this purpose is designed to 


deliver a small current at high potential and has a 
large leakage reactance and a high secondary re¬ 
sistance, both of which are utilized to limit the 
charging current to the 0.1 pf flash lamp condenser. 
A Variac is used to control the voltage applied to 
the primary of this transformer and to control 
thereby the rate at which the lamp is flashed. If the 
voltage is sufficiently high, the condenser will be 
charged rapidly enough to fire the lamp once during 
each cycle. At somewhat lower voltages the flashing 
rate will be less, and at much higher voltage, 
greater. However, the most stable and regular oper¬ 
ation occurs at the rate of 60 per second or an inte¬ 
gral fraction thereof. 

6 2 5 Field Test Results 

Tests of Army Model 

The Army model of the IRRAD equipment was 
demonstrated at Fort Belvoir, Virginia, on the 
nights of July 28 and August 8, 1944, to Army, 
Navy, British, and NDRC representatives. Since 
further work on the battery-operated power supply 
was still required, an a-c power supply was used. 
The targets were triple mirrors of 40-millimeter 
aperture. 

In the first test it was demonstrated that indi¬ 
vidual targets dispersed in a valley containing trees, 
shrubs, and other objects could be readily detected 
with a few search sweeps; that the signal pulse was 
reduced to half-amplitude by a 2-minute shift in 
azimuth setting; and that an individual target could 
be detected and accurately ranged at 4,186 yards 
using Wratten 87 filter, or at 2,089 yards using 
Polaroid XR7X26 filter in the transmitter. 

Tests on the accuracy of range and azimuth 
measurement were made on the second night. Two 
observers rapidly located six individual targets at 
ranges up to 2,100 yards and measured their range 
and angle coordinates. The errors of observer A 
were all within 5 yards in range and 3 minutes in 
azimuth; those of observer B, who had had no 
previous experience with the equipment, were within 
14 yards and 10 minutes, respectively. Since B’s 
range errors were all positive, it is thought that his 
readings may have been consistently large due to 
the fact that he was not invited to make his own 
zero set, as discussed in Section 6.2.2, “Range and 
Sweep Circuits.” 






210 


IRRAD SYSTEMS 


Tests of Navy Model 

Field tests of the Navy model, essentially identi¬ 
cal with the Army model except for the a-c power 
supply, were made in cooperation with the Navy on 
October 23 and 24, 1944, at the Bureau of Ships 
Test Station, Fort Miles, Delaware. The IRRAD 
equipment was set up on the shore. A ship, 
equipped with a crown of six triple mirrors around 
a mast to provide a suitable target at all headings, 
made trips over an agreed course while range- 
azimuth-time measurements were made at about 
100-yard intervals out to the 4,500-yard maximum 
range of the calibrated dial. The maximum range 
at night using Wratten 87 filter in the transmitter 
exceeded the 4,500-yard calibration limit and was 
estimated from the course and time measurements 
to be about 6,000 yards. With no filter, successful 
observations were made to an estimated range of 
about 7,500 yards, the signal-to-noise ratio being 
approximately two to one at this distance. The 
atmospheric transmission was estimated to be about 
70 per cent per mile. When the source was covered 
with Wratten 87 filter, it was invisible beyond 
about 400 yards to an unaided, dark-adapted eye, 
but could be seen to nearly 800 yards with 7-power, 
50-millimeter binoculars. When the XR7X26 filter 
was used, the source was completely invisible at any 
range. 

Successful daytime tests were also made along 
the shore line in bright sunlight. Using Wratten 87 
filter over the source, ranges were measured up to 
the maximum unobstructed distance of 2,100 yards. 
At this distance the signal-to-noise ratio was about 
five to one. 

626 Discussion 

Tungsten or Mercury Source 

Since it is possible by means of mechanically 
operated components to project short pulses of radi¬ 
ation from a continuously emitting source, experi¬ 
mental systems of this type were constructed and 
tested before the microflash lamp which was finally 
adopted had become available. A d-c operated in¬ 
candescent tungsten lamp was used as the source in 
two such systems. The first employed a rapidly 
rotating disk in which narrow slits were cut, and 
the second employed one rapidly rotating mirror 
and several fixed mirrors in conjunction with a 


stationary slit. A high-pressure mercury vapor lamp 
was also tried as the source in the second system. 
Although some measure of success was obtained in 
each case, the accuracy obtainable in range and 
angle settings never approached that which was 
obtainable through the use of the short-duration 
flash lamp. Moreover, it was never possible to 
establish with the tungsten or mercury source in 
either system an operating range comparable with 
that expected on the basis of a comparison of the 
holobrightness (see the Appendix) of these sources 
with that of the flash lamp. Although a part of this 
discrepancy may have been due to vibrations ex¬ 
cited by the high-speed rotating elements, the rea¬ 
sons for some of the difficulties encountered experi¬ 
mentally have not been fully established and do 
not merit further discussion here. 

Final Status of Army Model 

Following the NDRC demonstration and addi¬ 
tional tests conducted by Army personnel, a report 4 
on the IRRAD equipment was issued by the Engi¬ 
neer Board of the Army Corps of Engineers. 
Although the experimental model essentially met 
the military characteristics for which it was con¬ 
structed, it was concluded that it did not meet 
satisfactorily the accuracy and dependability re¬ 
quirements for field use in surveying and that its 
use in other applications would be limited. In view 
of the long period which would be required to 
develop the equipment in such a way as to meet the 
requirements for widespread field use as a portable, 
high-precision surveying instrument, and the spe¬ 
cial training which maintenance men would need, 
it was recommended that the development be con¬ 
tinued by NDRC or other civilian research agency 
on a long-term, low-priority basis. The principal 
points of interest for further development were 
stated to be improved accuracy as a surveying 
instrument, reduction of size and weight to permit 
easier transport in manborne packages, and any 
possible increase in ruggedness and simplicity for 
field use. No additional development along these 
lines was undertaken by NDRC. 

Final Status of Navy Model 

Because of continuing Navy interest, some ex¬ 
ploratory work on a plan-position-indicating adap¬ 
tation was conducted under NDRC auspices. The 
relations between pulse rate, rotational scanning 


■ l TJflm -n-rrrregi^ 
14 in I IM7 



IRRAD FOR DIFFUSELY REFLECTING TARGETS 


211 


rate, and azimuth angle of the projected beam were 
investigated. The persistence characteristics of the 
cathode-ray-tube screen demanded that the pattern 
be reproduced at least once in every four seconds. 
The pulse rate was increased to 120 flashes per 
second and all slits were removed from the trans¬ 
mitter in order to utilize the entire width of the 
flash-lamp discharge, subtending about 1.5 degrees 
in azimuth. Since the beam subtends only 2.5 de¬ 
grees in the vertical plane, it was found necessary 
to mount the scanning head on a gyrostabilized 
platform. Optical elements of both the transmitter 
and the receiver were then redesigned to reduce the 
weight of the scanning head. At the same time 
improvements were incorporated which promised to 
extend the feasible operating range to 10,000 yards 
at night when using the Wratten 87 filter. A circuit 
was also devised to vary automatically the gain of 
the photomultiplier tube in such a way as to com¬ 
pensate for variations in signal intensity with range 
and to achieve nearly equal brightness of the indi¬ 
cating spot on the screen of the cathode-ray tube 
for targets at all ranges. The special components 
required for a shipborne installation utilizing this 
type of signal presentation were furnished by the 
Navy, and the completion of a test model was taken 
over by a direct contract, superseding Contract 
OEMsr-1267. The details of construction and oper¬ 
ation of the completed model were not available for 
inclusion in this report. 

Recommendations for Further Development 

In view of the high precision in range and direc¬ 
tion measurements demonstrated to be possible 
with existing IRRAD equipments, it is recom¬ 
mended that refinements in the optical systems and 
in the mode of aligning the index pulse with the 
signal pulse be added for any adaptation in which 
such refinements would result in an equipment 
having considerable value for military purposes at 
ranges somewhat in excess of 4,000 yards for a 
single triple-mirror target. Questions pertaining to 
type of power supply, ruggedness, divisibility into 
manborne packs and total size and weight limita¬ 
tions should be jointly considered in the light of 
future military needs. Some improvements appear 
to be possible in each instance, although successful 
major alterations in these details will be contingent 
upon the development of greatly improved com¬ 
ponents such as flash lamps, photomultipliers, etc. 


63 IRRAD FOR DIFFUSELY 

REFLECTING TARGETS 

Equipment for investigating the practicability of 
an IRRAD system for use in the detection of large, 
diffusely reflecting objects, such as ships against a 
sea background, was constructed and tested 5 under 
Contract NDCrc-185. The close liaison already 
established between this contract and Contract 
OEMsr-1267 for the development and testing of a 
suitable microflash lamp source was continued for 
this development. Since the principles of operation 
and the required components are essentially iden¬ 
tical in the two types of IRRAD systems and since 
the equipment for detecting diffusely reflecting 
targets was never carried beyond the laboratory 
experimental stage, only a brief description of the 
latter equipment will be given with the points of 
difference rather than of similarity between the two 
systems emphasized. 

With an extended target having a surface of low 
reflectance it becomes necessary to abandon high 
precision of azimuthal angle settings as an objective 
and to adopt every possible measure which will 
increase the amount of flux impinging on the target 
and, therefore, on the detecting element after reflec¬ 
tion, in order to achieve operation at a large enough 
range to be of military value. This difference arises 
from the fact that the small, highly efficient, triple¬ 
mirror target can utilize only that flux which is 
projected into the very small solid angle defined at 
the transmitter by the target, which subtends no 
more than a few seconds of arc at threshold range; 
while for an extended target all the flux projected 
into a zone subtending as much as several degrees 
of arc may contribute to the integrated intensity of 
the reflected signal. Thus an optical system having 
a low //number is advantageous, the focal length 
being chosen with due regard to the dimensions of 
the source and the estimated angular extent of the 
target. 

631 General Description of Equipment 

The source used in the transmitter was a type 300 
microflash lamp (see Section 1.3.1), with the arc 
centered at the focal point of a precision, glass, 
second-surface parabolic reflector of 19% 6 -inch 
aperture and 7%-inch focal length. Lamps having 
an arc length of approximately 1 centimeter were 





212 


IRRAD SYSTEMS 


used, oriented with the arc gap transverse to the 
mirror axis. The angular dimensions of the projected 
beam were approximately 1x3 degrees, with the 
long dimension horizontal. The lamp was mounted 
directly on the firing condenser and was enclosed in 
a coaxial cylindrical copper shield provided with a 
suitable optical aperture facing the mirror. The 
capacitance of the firing condenser was 0.1 pf. The 
lamp was fired 60 times per second at its self- 
breakdown potential, using an unfiltered half-wave 
rectifier as the power supply. Either a Wratten 87 
or a Polaroid XR3X44 infrared transmitting filter 
could be inserted in the cover of the reflector hous¬ 
ing. The peak candlepower at the center of the 
beam, measured through the Wratten 87 filter with 
reference to a cesium-surface phototube, was about 
7.5 X 10 7 equivalent holocandles. 

The receiver optical system consisted of a semi¬ 
precision metal parabolic reflector of 18-inch aper¬ 
ture and 7%-inch focal length, with the cathode of 
a 14-stage, end-view, cesium-cathode photomulti¬ 
plier tube (constructed under Contract OEMsr- 
1094) located in the focal plane. Because of the 
construction of the cathode the angular width of the 
receiver field of view was limited to 1.5 degrees or 
less over a major portion of the mirror aperture. 
An 11-stage tube having a cathode and envelope 
arrangement which was more efficient for this appli¬ 
cation was later constructed under Contract 
OEMsr-1094 but was not available for use at the 
time of the field tests on actual ship targets. 

The output of the multiplier was fed through a 
high-gain wide-band amplifier to the vertical de¬ 
flection plates of a 5-inch cathode-ray tube. Both 
the amplifier and the cathode-ray tube power sup¬ 
ply were obtained from the Radiation Laboratory 
of the Massachusetts Institute of Technology, hav¬ 
ing been originally developed as components for the 
P-4 synchroscope. A triggered high-speed sweep 
circuit with trace-brightening voltage generator, 
constructed according to specifications obtained 
from Contract OEMsr-1267, was used to control 
the horizontal deflection and the brightness of the 
trace. No timing and range pulse circuits were 
included in this equipment. The target range was 
determined from the distance between a zero range 
index and the target pulse on the screen of the 
cathode-ray tube, a chart for this purpose having 
been prepared for each sweep speed. Any one of five 
sweep speeds between 0.75 and 12.3 psec per inch 


could be selected at will, as found convenient for 
measuring the ranges of targets at different dis¬ 
tances. The accuracy of range measurements was 
about ±5 per cent. No provision was made for the 
measurement of angles of azimuth or elevation, 
although circles for making approximate measure¬ 
ments of these angles could easily have been added. 
Also, no attempt was made to hold the weight, size 
or power consumption of the equipment to minimum 
values, since the project was on a short-term, ex¬ 
ploratory basis. 



Figure 5. Complete IRRAD equipment for diffusely 
reflecting targets. Left to right: sweep and trace- 
brightening generator, oscilloscope, multiplier power 
supply, scanning unit, lamp power supply. 

The complete experimental apparatus is shown 
in Figure 5. The optical components of the trans¬ 
mitter and the receiver were bolted rigidly to an 
iron framework, thus forming a single “scanning 
unit” which could be rotated about either a hori¬ 
zontal or a vertical axis. A Polaroid sight was used 
to check the approximate orientation. The weight 
of the entire equipment was approximately 450 
pounds. All components were designed to operate 
from a 115-volt, 60-cycle power line. 












IRRAD FOR DIFFUSELY REFLECTING TARGETS 


/ 


213 


6 3 2 Field Test Results 

Preliminary tests were conducted on the Univer¬ 
sity of Michigan campus with targets consisting of 
a few relatively distant structures to which an 
unobstructed view could be obtained. Photographs 
of traces in which the presence of these stationary 
targets was indicated are shown in Figure 6. It was 



ABC 


Figure 6. Typical oscilloscope patterns (7/50 actual 
size) for diffusely reflecting targets. A. Target con¬ 
sisted of a pair of red brick chimneys 1,380 and 
1,560 feet distant. The exposure includes 120 traces 
at 0.93 psec per inch. B. Target consisted of a pair 
of red brick chimneys 1,380 and 1,560 feet distant. The 
exposure includes 1,800 traces at 3.3 p,sec per inch. 
Note the increase in backscattering relative to the 
target echo as compared with A which was photo¬ 
graphed on a different evening. C. Target consisted 
of a part of the University of Michigan Hospital, 
a white limestone structure with large window areas 
2,525 feet distant. The exposure includes 60 traces at 
3.3 nsec per inch. This was photographed a few min¬ 
utes after A. 

found possible to resolve two targets which were 
simultaneously in the field of view, separated by 
only about 12 per cent of their mean distance of 
1,470 feet from the equipment, and to measure the 
range of targets at distances from 1,100 to 2,500 
feet with an accuracy of about ±5 per cent of the 
distances obtained from a scale map of the campus. 
These tests were adversely affected by the large 
amount of stray radiation from nearby lights to 
which the photomultiplier cathode was exposed and 
by the smoke and haze generally encountered dur¬ 
ing the tests. The high peak at the left of each trace 
is due to radiation scattered back to the receiver 
from the atmosphere and nearby objects. There is 
some evidence for the existence of a time delay in 
the recovery of the photomultiplier tube from over¬ 
loading due to the initial backscattering. This delay 
is long enough to increase the minimum detectable 
signal, even at the time the target echo pulse is 
received, due to the persistence of the abnormally 


high noise level associated with large values of 
multiplier current' The adverse effects of back- 
scattering could presumably have been diminished 
if the transmitter and receiver optical systems had 
been a good deal more widely separated in space. 
This condition could only be achieved with a much 
more complicated system, which was not feasible 
in this experimental equipment. 

On the night of October 10, 1944, ship detection 
tests were made from a station on the shore of 
Lake St. Clair at a point about 15 feet above the 
water level, using Great Lakes freighters as targets. 
Their hulls are invariably dark, with an estimated 
reflectance of only about 5 per cent. However, at 
bow and stern there are small superstructures which 
are ordinarily painted white and have an estimated 
reflectance of about 75 per cent. The average height 
above the water line may vary from about 12 to 
25 feet, depending on the ship and how heavily it is 
loaded. The length of a ship may be from about 450 
to more than 600 feet. Since the feasible range of 
detection for these targets was only slightly more 
than one nautical mile, the length of each ship at 
broadside aspect subtended more than the 3-degree 
horizontal extent of the beam, while the average 
height of each target was sufficient to fill only one- 
fourth or less of the vertical extent of the beam. 
As the equipment was scanned over the horizontal 
plane it was found that the signal indication was 
much stronger at two slightly separated orienta¬ 
tions, evidently those at which the beam included 
either the fore or the aft superstructure, than at 
intermediate orientations for which neither white- 
painted superstructure was included in the beam. 

Measurements of the range of detection were 
successfully made on eleven ships, using several 
combinations of flash-lamp source and photomulti¬ 
plier detector tube. The shortest observed target 
range, set by the distance from the test station to 
the ship channel, was 5,200 feet. Even at this mini¬ 
mum feasible test distance it was not possible to 
detect a ship when the Polaroid XR3X44 filter was 
over the transmitter. The maximum observed range 
of detection with the Wratten 87 filter was 6,800 
feet; with no filter, 7,500 feet. The increase in range 
obtained by removal of the filter is thought to have 
been limited principally by overloading of the photo¬ 
multiplier tube due to radiation backscattered from 
the atmosphere, since a much greater increase in 
range would have been expected on the basis of the 





214 


IRRAD SYSTEMS 


ehT value of the filter, atmospheric attenuation and 
distance effects. However, other conditions of the 
tests could not be closely controlled. For example, 
it was not possible to identify each ship in order to 
determine its size, nor to tell whether or not it was 
heavily loaded with cargo. The changing aspect 
presented by the ships as they moved past the test 
station along the navigation channel constituted 
another variable factor not fully accounted for. The 
possible effects of ship lights on the noise level of 
the photomultipler tube, and of ship smoke on the 
ranges of detection, should also be considered. The 
importance of the latter effect is illustrated by the 
twice-observed fact that it was not possible to detect 
the more distant of two ships which passed each 
other almost directly off the test station, presum¬ 
ably because of the extra attenuation introduced 
by the smoke trail of the nearer ship. 

The visual range of the transmitter when covered 
with Polaroid XR3X44 filter was between 150 and 
1,100 feet; with Wratten 87 filter it was consider¬ 
ably in excess of 1,100 feet. The project was termi¬ 
nated before an accurate threshold visual-range de¬ 
termination was made with either filter. 

633 Discussion and Evaluation 

The results of the detection tests on lake freight¬ 
ers were used as a basis for estimating the probable 
results on military vessels. The estimated range of 
detectability for a destroyer, using a Wratten 87 
filter on the equipment, is about 7,000 feet. The 
effective operating range which might be secured by 
incorporating a number of possible refinements and 
using components of superior quality was thought 
to be about 2 nautical miles for a destroyer and 3 
for a large battleship. 

The possible effectiveness of using retrodirective 
reflectors of the highway-marker type as targets 


for this equipment was also considered. A battery 
of 12 such reflectors, each 3 inches in diameter, 
should give approximately twice the range of de¬ 
tection for a destroyer, or about 4 nautical miles. 
Installations of these reflectors would be particu¬ 
larly effective in making possible the detection of 
small ships and boats, marker buoys, etc. However, 
the only advantage of these reflectors over the high- 
precision Mt. Wilson triple mirrors, used as targets 
for the equipment described in Section 6.2, is their 
relatively low cost and ease of procurement. With 
the less precise reflectors of the highway-marker 
type the IRRAD equipment needed to provide equal 
operating ranges is heavier, bulkier, and probably 
more costly than would be justified by the differ¬ 
ence in cost of the two types of reflectors. 

The relatively short operating ranges indicated by 
the field tests described above, the low estimates 
based thereon for the range of detection of military 
vessels, and the low visual security afforded by the 
experimental apparatus when the Wratten 87 filter 
was used did not appear to warrant additional de¬ 
velopment, tests, and demonstrations. Bureau of 
Ships personnel concurred with NDRC representa¬ 
tives in the opinion that the limited ranges of suc¬ 
cessful operation which can be obtained with 
IRRAD equipment of reasonable weight and size 
do not have potentially significant military value. 
The project was therefore terminated without an 
official demonstration for the Armed Services. It is 
recommended that the future reopening of such a 
project be contingent upon the development of new 
and improved components which might offer the 
possibility of increasing the range of detection by 
an order of magnitude. In the absence of such new 
developments, further work on IRRAD systems 
should be confined to types designed for the detec¬ 
tion of retrodirective reflectors of high accuracy, as 
described in Section 6.2. 






Chapter 7 

GLIDER POSITION INDICATOR AND 
CLOUD ATTENUATION METER 

By Thomas R. Kohler a 


71 INTRODUCTION 

E arly in the use of towed gliders as troop or 
materiel transports it became apparent that a 
blind-flying system, which would enable the glider 
pilot to maintain the correct position behind the 
tow plane, w’ould be extremely useful. Accordingly, 
at the request of the Army Air Forces, such a sys¬ 
tem was developed at the University of Michigan 
under Contract NDCrc-185 as Project Control 
AC-56. 

7,1,1 Analysis of the Problem 

The Army Air Forces sought an infrared device 
for indicating, within ±2 degrees, the position of a 
glider with respect to its tow plane 400 feet away 
through clouds. This problem might be solved by a 
device which would indicate the position of the 
glider relative to the tow plane directly (a positional 
system), or by a device which would operate from 
the heading and flight attitude of the glider with 
respect to the tow plane (a directional system). 
With the directional system the glider might be in 
the proper position, but the device would indicate 
“out of position” if the glider were not pointing in 
the proper direction. Conversely, under this system 
it would be possible for the glider to be “out of 
position” and be indicated as “in position.” A posi¬ 
tional system, on the other hand, would enable the 
pilot to know a,t any instant the position of the 
glider with respect to its desired position regardless 
of its flight attitude. If the glider began to deviate 
from the desired position, this system would indi¬ 
cate directly the magnitude and direction of the 
deviation. The positional system seemed, therefore, 
to be the obvious and best solution to the problem. 
The glider position indicator [GPI] 1 is a true 
positional system. 

a Department of Engineering Research, University of 
Michigan. 


Atmospheric Attenuation 

In any equipment such as this, the range of 
operability is critically dependent on the atmos¬ 
pheric attenuation to which the signal radiation is 
subjected between the transmitter and the receiver. 
Thus, if the transmitter at a fixed distance from the 
receiver would produce at the receiver an illumina¬ 
tion E 0 with only clear air between, and an illumi¬ 
nation E with a certain cloud between, the relation 
between these two values is: 

E = E 0 e~ kx , 

where x is the distance between transmitter and 
receiver and k is the attenuation coefficient of the 
cloud. The decibels of attenuation would then be 
expressed as: 20 kx logio e or 8.7 kx. The factor 20 
is used, since it is the voltage rather than the power 
output of the receiver which is proportional to the 
radiant energy arriving at the receiver. 

In the case of near infrared radiation, the attenu¬ 
ation coefficient is known to be very nearly the same 
as for visible light, but no data were available, in 
usable form, on the limiting values to be expected 
in dense clouds. For this reason a cloud attenuation 
meter [CAM] 2 was designed and built to investi¬ 
gate the values of the attenuation coefficient likely 
to be encountered in dense clouds. In this investiga¬ 
tion, the information desired may be divided into 
three categories: (1) the cloud attenuation values 
existing between the transmitter and receiver of 
the glider position indicator at any given time; (2) 
the maximum cloud attenuation values which might 
be encountered; (3) a correlation between the cloud 
attenuation coefficient and the visual range. Item 
(1) would establish the performance characteristics 
of the GPI model actually constructed with refer¬ 
ence to operation through clouds. Item (2) would 
establish how powerful the GPI would have to be 
in order to operate through any cloud condition that 


RESTftiGTED 


215 



216 


GLIDER POSITION INDICATOR AND CLOUD ATTENUATION METER 


might be encountered. Item (3), if such a correla¬ 
tion exists, would make possible the determination 
of the attenuation coefficient by a very simple 
measurement which could be made at almost any 
location with no auxiliary equipment, since the 
visual range is merely the limiting range at which 
a large dark object may be seen on the horizon. 
The relationship 3 between visual range and the 
attenuation coefficient is given by: 

Xm = l log, t 

where 

X m = visual range, 
k — attenuation coefficient, 

8 = threshold contrast. 

Values of £ computed by means of this equation, 
from experimentally determined simultaneous val¬ 
ues of X m and k, appear to indicate that £ is not 
constant under all conditions but may vary from 
0.02 for visual ranges of several miles to 0.065 for 
visual ranges of approximately 100 feet. 

While the CAM was originally designed specifi¬ 
cally to measure the attenuation of near infrared 
radiation by dense clouds over short ranges, it could 


easily be adapted for general studies of the trans¬ 
mission of visible near infrared radiation by the 
atmosphere. 

72 GLIDER POSITION INDICATOR 

7,2 1 General Description and Mode of 
Operation 

The GPI consists essentially of a near infrared 
transmitter, mounted on the tow plane, which scans 
a 20x20-degree field in which the glider is to fly, 
and a receiver, mounted in the nose of the glider, 
utilizing a photoconductive thallous sulfide cell as 
the radiation detector. The receiver circuits are 
synchronized with the transmitter scan by means 
of a cable along the tow rope in such a way as to 
indicate the glider's position in the field scanned 
by the transmitter. 

As shown in Figure 1, the transmitter projects a 
narrow beam of radiation in the general direction 
of the glider. This radiation is given a compound 
oscillatory motion by means of two vibrating mir¬ 
rors with sinusoidal motions at right angles. The 
result is a Lissajous figure of frequency ratio 6 to 1 



GLIDER DISPLACED TO RIGHT 


SEC aa’ of end appearance 

RADIATION FIELD OF INDICATOR TUBE 

Figure 1 . Operation of infrared glider position indicator. 































GLIDER POSITION INDICATOR 


217 


in that the beam is swept across the field 6 times in 
the horizontal direction for each one in the vertical. 
The complete scanning process is repeated 14 times 
a second. 

The glider pilot has before him a cathode-ray 
oscilloscope, the electron beam of which is driven 
exactly in synchronism with the scanning radiation 
beam from the transmitter. This is accomplished 
by means of electrical pickups at the transmitter 
mirrors which deliver the synchronizing voltages to 
the receiver through the cable along the tow rope. 
Thus the trace of the transmitted radiation beam 


‘ 2 2 Description of Component Parts 

Transmitter 

The optical system of the transmitter consists of 
a tungsten lamp, the radiation from which is focused 
by a 3-inch parabolic mirror of 2 inches focal length 
into a beam subtending 1 degree by 5 degrees. The 
beam is reflected from the first vibrating plane 
mirror to the second, and then to the zone in which 
the glider is to be located. The two vibrating mir¬ 
rors impart to the beam the scanning motion de- 



Figurb 2. Components of the transmitter. Left to right: monitor oscilloscope, control cabinet, source-optical 
system. 


on a vertical plane through the glider’s nose and the 
trace of the electron beam on the screen of the 
cathode-ray oscilloscope describe similar patterns 
and remain exactly in step while doing so. However, 
the intensity of the trace on the screen is sup¬ 
pressed below the visual threshold, except at that 
moment when the transmitted radiation beam passes 
over the detector cell located in the nose of the 
glider. The electrical impulse from the cell is ampli¬ 
fied and is made to energize or “trigger” the spot on 
the oscilloscope screen up to a visible level at just 
this time, so that the spot makes its appearance at 
just that place in the electron beam pattern on the 
screen which corresponds to the glider’s position in 
the radiation beam pattern. This arrangement will 
be recognized as a rudimentary television system 
with the radiation pickup in the screen itself. 


scribed in Section 7.2.1. The optical portion of the 
transmitter is 11x15x15% inches and weighs 56 
pounds. This size can not be greatly decreased with¬ 
out a corresponding sacrifice in size of the projected 
field. In addition, the transmitter includes a control 
cabinet, containing amplifiers for the mirror drives, 
and a monitor oscilloscope. Figure 2 shows the com¬ 
ponents of the transmitter, and Figure 3 is a block 
diagram showing the various amplifiers and phase- 
shift circuits of the transmitter. The total weight 
of the transmitter is 206 pounds. This weight could 
be considerably reduced, as the monitor oscilloscope 
probably would not be necessary and the control 
cabinet could be made considerably smaller and 
lighter than in the experimental model. The trans¬ 
mitter is operated from a 110-volt a-c and a 12-volt 
d-c supply. 















218 


GLIDER POSITION INDICATOR AND CLOUD ATTENUATION METER 


EXT OSC 



no v 
60 ~o 


Figure 3. Block diagram of transmitter circuits. 


Receiver 

The optical “eye” of the receiver utilizes a spheri¬ 
cal mirror to collect the radiation from the trans¬ 
mitter and to concentrate it on the radiation-sensi¬ 
tive thallous sulfide photoconductive cell. The 
combination of cell and mirror which was used gave 
a field of view subtending 20x20 degrees. The elec¬ 
tric signal from the phototube passes through a pre¬ 
amplifier, an amplifier, and a differentiating circuit 
to a trigger circuit which controls the intensity of 
the spot on the cathode-ray tube. This sequence is 
indicated in the block diagram of the receiver shown 
in Figure 4. The cathode-ray oscilloscope cabinet 
also contains amplifiers for the sweep synchronizing 
voltages obtained from the transmitter through the 
connecting cable to the tow plane. The components 
of the receiver are shown in Figure 5. 

Operating Tests 

Ground Tests 

Preliminary tests were made in clear weather on 
the University of Michigan laboratory roof during 
July 1943 to determine the sensitivity of the equip¬ 
ment under both day and night conditions. It was 



Figure 4. Block diagram of receiver circuits. 


determined that, with 200 feet separating the trans¬ 
mitter and receiver, the receiver response was ap¬ 
proximately 45 db above threshold; that is, about 





























































































































































































GLIDER POSITION INDICATOR 


219 


45 db of attenuation could be tolerated. The amount 
of tolerable attenuation showed no marked depend¬ 
ence on sky brightness, which varied from zero to 
250 candles per square foot during these tests. 

The GPI was tested during August 1943 on the 
summit of Mt. Washington, New Hampshire, where 
the effect of actual clouds on the operation of the 
equipment could be determined. The CAM was used 
to measure the cloud attenuation. 


tions were thick enough for a stringent trial of the 
equipment, tests 'were made in clear air. 

Four flight tests were conducted, at altitudes up 
to 10,000 feet. A standard tow rope (350 feet long) 
was used throughout the trials. The various glider 
pilots experienced no difficulty in maintaining the 
glider in its correct position by means of the indi¬ 
cations from the GPI. The equipment operated 
satisfactorily without attention or adjustment. So 



Figure 5. Components of the receiver. Left to right: detector-optical system (receiver eye); preamplifier 
(below) and amplifier (above); oscilloscope cabinet containing sweep amplifiers, trigger circuit, and power 
supply for cathode-ray tube; cathode-ray tube for signal presentation. 


The GPI was found to be capable of penetrating 
36 decibels of cloud under daylight conditions, with 
the transmitter and receiver 217 feet apart. Such a 
cloud would have an attenuation coefficient k of 
1.9 per 100 feet. At night the GPI could penetrate 
about 45 db of cloud, or a cloud with an attenua¬ 
tion coefficient of 2.4 per 100 feet. These cloud 
attenuation values would be approximately equiv¬ 
alent to visual ranges of 144 feet and 114 feet, re¬ 
spectively. 

Flight Tests % 

The GPI was flight-tested at the Clinton County 
Army Air Base near Wilmington, Ohio, during 
November 1943. Because Army flight rules did not 
permit flight during those times when cloud condi- 


far as it was possible to ascertain from observations 
and comments made by the pilots, the GPI possessed 
the essential operating characteristics originally re¬ 
quested. That is, it indicated the position of the 
glider at all times when the glider was in its assigned 
20x20-degree field, with a horizontal resolving power 
considerably better than the minimum requirement 
of two degrees. It was somewhat deficient in vertical 
resolving power (2.5 degrees instead of the required 
2.0 degrees). It must be noted, however, that the 
position indication spot blanked out during opera¬ 
tion of the plane’s radio, a circumstance which 
could perhaps be eliminated by additional shielding 
of the radio transmitter or the GPI receiving cir¬ 
cuits (see Section 7.2.4). No change in performance 
with change in altitude was detected. 









220 


GLIDER POSITION INDICATOR AND CLOUD ATTENUATION METER 


7 2 4 Discussion of Test Results and 
Recommendations 

Experience gained during the various operating 
tests, together with the comments of the pilots, indi¬ 
cate the desirability of the following improvements, 
all of which appear to be feasible: 

1. Increase the field scanned by the transmitter 
and the field of view of the receiver eye from 20x20 
to 30x30 degrees. 

2. Make the scanning pattern finer by increas¬ 
ing the number of horizontal sweeps per vertical 
sweep. This would increase the resolving power in 
the vertical direction. 

3. Provide an indicator and manual controls or 
completely automatic controls for amplitudes of the 
vibrating mirrors in the transmitter, in order to 
maintain any required angular dimensions of the 
scanning field at all times. 

4. Provide the shielding necessary to isolate the 
position-indicator circuits from the plane’s radio. 

5. Provide a control for aiming the transmitter 
in the vertical plane, adjustable in flight. This 
would permit adjustment of the equipment to allow 
for any desired change in the preferred glider posi¬ 
tion during an actual flight. 

6. Make the position-indicating spot on the 
cathode-ray tube screen very much brighter. 

The tests made through real and simulated clouds 
show that the GPI would be some 7 to 16 db defi¬ 
cient in penetrating power through the densest 
clouds, even at a separation of only 200 feet be¬ 
tween glider and tow plane. This is assuming that 
flight is possible through clouds having an attenua¬ 
tion coefficient k of as much as 3.0 per 100 feet. The 
improvement demanded to achieve operation at 
200 feet under these conditions is not beyond what 
might be achieved in an improved model, incorpo¬ 
rating refinements over the first experimental 
model described here. However, to achieve opera¬ 
tion at 400 feet through such clouds or at distances 
of 200 feet or more through very much denser 
clouds (if they exist under possible flight condi¬ 
tions) would require a penetrating power more than 
an order of magnitude better than the experimental 
model. This cannot be achieved by any method now 
apparent for equipment of this type. If and when 
the further development of such a system may be 
considered, the following questions will be crucial: 
(1) How short a tow rope may be contemplated. 


(2) What maximum cloud attenuation must be 
provided for. 

The results of the flight tests in clear air indicate 
that, apart from cloud-penetrating power, the pres¬ 
ent model has the required essential operating char¬ 
acteristics. Another model could include the six 
recommendations previously outlined and, in addi¬ 
tion, be made much lighter, more compact, and 
require less power. It does not appear feasible, 
however, to reduce materially the size of either the 
transmitter optical system or the receiver “eye,” 
or to dispense with the three-wire cable along the 
tow rope in a system of this type. 

725 Present Status 

Although the field tests of the GPI showed that 
it possessed the general operating characteristics 
originally requested for military applications and 
the feasible modifications which would improve its 
practical usefulness were clearly outlined, the Army 
Air Forces at that time decided not to request any 
further development. 

73 CLOUD ATTENUATION METER 
General Description 

The CAM consists of a control-meter unit [CMU] 
and a reflector-modulator unit [RMU] located at 
a chosen distance from a source-detector unit 
[SDU]. These are shown in Figure 6. In operation, 
as shown in Figure 7, the SDU projects a beam of 
radiation through the chosen optical path to the 
RMU, which retrodirectively reflects a portion of 
the beam to the detector in the SDU and at the same 
time modulates it at 90 cycles per second. The 
detector then measures, by means of a tuned ampli¬ 
fier and metering circuit, the returned radiation. 
Comparison of such a measurement made through a 
cloud with a measurement made through clear air 
allows one to compute a value for the attenuation 
coefficient k of the cloud. A means for standardizing 
readings has been included in the SDU in order to 
make an absolute calibration unnecessary each time 
the instrument is used. 

7,3,2 Description of Component Parts 

Source-Detector Unit 

The radiation source is an 85-watt, 13-volt, tung¬ 
sten flashing-signal lamp. Mounted coaxially behind 



CLOUD ATTENUATION METER 


221 



Figure 6. Cloud attenuation meter. Left to right: source-detector unit, control-meter unit, reflector-modu¬ 
lator unit. 



Figure 7. Schematic diagram of cloud attenuation meter. 










































































222 


GLIDER POSITION INDICATOR AND CLOUD ATTENUATION METER 


the source is a parabolic mirror of 7%-inch aperture. 
The parts of an annular mirror which are not ob¬ 
scured by the parabolic one (see Figure 7) collect 
the radiation returned to the SDU by the RMU 
and directs it by means of an auxiliary plane 
mirror onto a diffusing glass screen behind which is 
mounted a thallous sulfide photoconductive cell. 
Also included in the SDU is an auxiliary light path 
consisting of three Lucite rod segments leading from 
the source to the detector. At the two spaces between 
these segments are inserted a motor-driven chopper 
for modulating the light and a magnetically con¬ 
trolled shutter. When the shutter is open, light is 
“conducted” from the lamp to the diffusing glass 
screen, being modulated in the process. Since the 
magnitude of this “internal signal” is independent 
of external atmospheric transmission conditions, it 
provides a reference signal to which all other read¬ 
ing may be referred in terms of decibel differences. 
This feature is considered especially important, as 
the “external” and “internal” readings are affected 
identically by changes in lamp characteristics, lamp 
voltage, photocell responsivity, amplifier gain, and 
unmodulated masking flux received either from day¬ 
light or by back reflection from the beam. With a 
thallous sulfide cell as the radiation detector a cor¬ 
rection for the effect of masking flux is essential. 
For this purpose it is advantageous to use a cell 
selected to have the least possible dependence of 
responsivity to masking flux. 

Reflector-Modulator Unit 

This unit consists of a group of three retrodirec- 
tive highway reflectors of standard commercial type 
centered at the corners of an equilateral triangle. 
A motor-driven modulator is mounted on an axis 
through the center of the triangle, its three blades 
successively covering and uncovering all three reflec¬ 
tors simultaneously. A shutter with three blades, 
mounted on the same axis and operated by an elec¬ 
tromagnet, may be thrown into either the “open” 
or “closed” position by means of a switch on the 
CMU. 

Control-Meter Unit 

This unit contains the amplifier and meter for 
measuring the output of the detector. The amplifier 
is a resistance-capacitance coupled type provided 
with a parallel-T negative feedback network for 
tuning it to 90 cycles per second, the modulation 


frequency. The gain of this amplifier is down 3 db 
at 87 and 93 cycles. The metering circuit is of the 
logarithmic vacuum-tube voltmeter type, so that 
the output meter reading is linear in decibels. 

In addition, controls for the shutter, motor, and 
lamp in the SDU and for the shutter and motor in 
the RMU are incorporated. Thus all controls are 
centralized in this one unit which can be installed 
at any convenient location within a reasonable dis¬ 
tance from either or both of the other two units. 

7 3 3 Theory of Operation 

To obtain the data required for computing the 
attenuation coefficient the shutters are operated in 
several ways. (1) With the shutter over the reflec¬ 
tors open and the shutter in the internal path 
through the Lucite rod closed, the meter will meas¬ 
ure the intensity of the radiation received from the 
retrodirective reflectors. This is the “external” 
reading. (2) With the shutter over the reflectors 
closed and the Lucite path open, the meter will read 
the “internal” or reference signal. (3) With both 
shutters closed a reading of the noise level of the 
equipment, which sets a lower limit for obtaining 
valid signal readings, may be made. 

In using the CAM, it is first necessary to obtain 
“clear” readings in the absence of appreciable at¬ 
tenuation in order to establish a reference level for 
the readings when fog or clouds are in the optical 
path between the SDU and RMU. The difference 
in decibels between the two readings is a direct 
measure of the amount of attenuation encountered 
over the entire path of the radiation from the source 
to the reflector and back to the detector. Since the 
“clear” and “cloud” readings may be separated by 
several hours, the necessity for the internal refer¬ 
ence signal is apparent. 

The loss of flux caused by the fog is given in 
terms of the attenuation coefficient by 



where I and I 0 are the amounts of reflected signal 
flux received by the detector through clear air and 
through fog, respectively, and y is the distance 
between the SDU and the reflectors. The attenua¬ 
tion caused by the fog is given in terms of deci¬ 
bels by 




CLOUD ATTENUATION METER 


223 


db F = 20 logio e~ 2kv 
= 40 logio e~ ky 
= — 17.4 ky, 

in which 7 and 7 0 are treated as voltages in the elec¬ 
trical circuit. The attenuation caused by the fog is 
given in terms of the meter readings by 

db F = (db e - dbi) + (dbi° - db e °), 

where db e = external fog reading, 
dbi = internal fog reading, 
db e ° — external clear reading, 
db° — internal clear reading. 

7 * 3 ' 4 Field Test Results 

In February 1943, a preliminary model of the 
CAM, designed to be installed in a PBY airplane, 
was taken to the Naval Air Factory in Philadelphia, 
Pennsylvania. Three weeks were spent at this time 
in making flights. No clouds suitable for measure¬ 
ment w r ere encountered at times when the weather 
conditions permitted flights to be made. It was de¬ 
cided, therefore, that future tests would have to be 
conducted on the ground at places where clouds 
were present much of the time. 

The CAM was set up temporarily at the summit 
of Mt. Washington in August 1943 when the GPI 
was tested there. Visual range measurements were 
made simultaneously with readings on the CAM. 
From the simultaneously measured values of the 
cloud attenuation coefficient and the visual range a 
probable value was obtained for e, the threshold 
contract factor introduced in Section 7.1.2, of about 
0.065. There was, however, quite a large spread 
above and below the figure in the individual values 
of e computed from the experimental data. The 
maximum attenuation coefficient that occurred dur¬ 
ing these tests was found to be 2.9 per hundred feet. 

In order to obtain more extensive data on the 
attenuation coefficients of clouds at visible and 
near-visible wavelengths, and also to obtain addi¬ 
tional information on the relationship between the 
attenuation coefficient and visual range, arrange¬ 
ments were made to install the CAM in the Mt. 
Washington Observatory and to have frequent 
observations and readings made over a period of 
months. 

In the data obtained from these tests, values of 
k up to 5.9 and of visual range down to 50 feet 
were observed. Whether or not these represent the 
limiting values which may be encountered in clouds 


is not definitely known, but it is certain that they 
will only infrequehtly be surpassed. 

7 3 5 Discussion of Field Test Results 

It is unfortunate that, because of the difficulties 
inherent in the measurement of atmospheric prop¬ 
erties, the data obtained from the Mt. Washington 
installation are not entirely consistent and so do not 
lead to explicit and reliable numerical relations be¬ 
tween visual range and the attenuation coefficient k. 
However, the data do show the following general 
characteristic trends. 

1. The experimental values of the attenuation 
coefficient k show a considerable spread in relation 
to visual range, the discrepancies becoming quite 
serious at the greater visual ranges. This is gener¬ 
ally to be expected, since the path of measurement 
extended somewhat less than 60 feet, and all but the 
shortest visual ranges had to be observed over a 
path distinctly different from that for which the 
attenuation was measured. 

2. The correlation between the attenuation co¬ 
efficient and the visual range apparently indicates a 
trend toward smaller values of e for larger values 
of visual range. This is in accordance with the 
results of previous work, 4 ’ 5 but the trend occurs at 
smaller visual ranges and is of much greater degree 
than would be expected from the earlier results. 

3. Although it had been hoped that the results 
would enable a choice to be made between the 
values of 0.02 and 0.065 for e (cf. Section 7.1), the 
indications are for a value intermediate between 
0.02 and 0.065 for visual ranges between 50 and 100 
feet, and for a value less than 0.02 for visual ranges 
between 100 and 150 feet. The latter indication, in 
particular, is hardly plausible and indicates the 
need for obtaining additional data under carefully 
controlled conditions. 

Sources of Error 

The determination of uniform and valid corre¬ 
sponding values of visual range and attenuation 
coefficient is unavoidably beset by difficulties which 
lead to errors. Some of these sources of error are 
given below. 

1. Differences between the path of instrumen¬ 
tal measurement and the path of visual range 
observation, both with regard to length and location. 
When the cloud is nonuniform, and particularly 







224 


GLIDER POSITION INDICATOR AND CLOUD ATTENUATION METER 


when there is considerable wind (as there invariably 
is on Mt. Washington), this is a possible source of 
error which becomes increasingly serious with in¬ 
creasing discrepancy in the length and location of 
the two paths. The steadiness or unsteadiness of the 
instrumental readings gives an index of this effect. 
Shielding by buildings and consistent vertical vari¬ 
ations in cloud density would introduce systematic 
errors. 

2. In the CAM itself, nonlinearity in response of 
the thallous sulfide cell could introduce a systematic 
error. Actually the response of thallous sulfide cells 
is known to be nonlinear at high signal levels, but 
this difficulty was minimized through careful selec¬ 
tion of the cell. Experimental checks on linearity 
of response versus signal intensity indicated entirely 
satisfactory performance of the CAM in this re¬ 
spect. 

3. Condensation of moisture on either or both of 
the windows covering the SDU and the RMU 
would lead to erroneously high values of the attenu¬ 
ation coefficient, k. 

4. It is perhaps possible (though it seems un¬ 
likely) that some change occurred in the decibel 
calibration of the output meter circuit upon which 
the decibel difference readings depend. It had not 
been possible to obtain a check on this calibration 
at the date of the contractor’s final report. 

Conclusions and Recommendations 

. In view of the apparent discrepancies in the data, 
it does not seem possible to draw any definite con¬ 


clusions with regard to either a maximum value for 
the attenuation coefficient of clouds found at the 
summit of Mt. Washington or a generally valid cor¬ 
relation of the measured values of the attenuation 
coefficients with the observed visual ranges. How¬ 
ever, the data tend to support the existence of a 
trend in this correlation, in a direction such as to 
require for very short visual ranges a higher con¬ 
trast between test object and surroundings than the 
classical value of 0.02. 

In order to obtain really satisfactory simulta¬ 
neous data on the attenuation coefficient and the 
visual range, the paths used for the attenuation 
measurement and the visual observation should be 
identical. This requirement is of course difficult, if 
not impossible, to meet exactly, but in future ex¬ 
perimental work every effort should be made to 
approximate it as closely as possible. 

Present Status 

At the conclusion of the experimental measure¬ 
ments on Mt. Washington, the CAM was trans¬ 
ferred to the Army Engineer Board, Ft. Belvoir, 
Virginia, for use in further investigations of the 
transmission characteristics of the atmosphere for 
near infrared radiation. It is anticipated that more 
complete information on these characteristics will 
continue to be of considerable interest and value in 
relation to actual or contemplated military equip¬ 
ments utilizing radiation in this wavelength region. 



/ 

\ 


Chapter 8 

FAR INFRARED DETECTING ELEMENTS 

By Harald H. Nielsen a and Alvin H. Nielsen b 


si INTRODUCTION 

A ll bodies which are at temperatures above abso- 
- lute zero emit heat radiation which is entirely 
similar to ordinary light except that the wave¬ 
lengths are much longer. These radiations are called 
infrared and are generally taken to embrace the 
wavelengths from 0.8 to about 400 p, where 1 p is 
10~ 4 centimeter. Radiation of this kind cannot be 
seen, and beyond 1.5 p cannot even be photo¬ 
graphed. For its detection, heat-sensitive devices 
must consequently be relied upon. 

For a body in equilibrium with radiation (a so- 
called black body), the wavelength at which the 
emission is a maximum is inversely proportional 
to the absolute temperature. For such a body, the 
maximum intensity of emission at that wavelength 



Figure 1 . Computed black-body energy curves for 
two bodies of equal size, one at 6000 K (Sun), the 
other at 287 K (Earth). 


is proportional to the fifth power of the absolute 
temperature. For example: the wavelength at which 
the emission of the sun (6000 K) is a maximum lies 
in the visible region (at about 0.5 p); while, at 
room temperatures, the maximum emission of a 
black body is at a wavelength some 19 times greater 
(9.5 p), and lies in the far infrared. The intensity 
of emission or surface brightness for the sun at 
0.5 p is approximately three million times (19 5 ) 
greater than it is for a black body at 27 C near its 
maximum of emission. This is shown in Figure l. 1 

a Mendenhall Laboratory of Physics, Ohio State Univer¬ 
sity, Columbus, Ohio. 

b Department of Physics, University of Tennessee, Knox¬ 
ville, Tennessee. 


The emission for a black body as a function of 
wavelength is given by Planck’s equation. This 
equation is plotted in Figure 2 for the temperatures 
50 C, 0 C, and -50 C. 



Figure 2. Atmospheric transmission curves for air 
path containing 0.25 cm precipitable H 2 0. 


Whereas the total radiation of a black body inte¬ 
grated over all wavelengths is proportional to the 
fourth power of absolute temperature, the spectral 
emission at ordinary temperatures, integrated from 
8.5 p to 13.5 p, is approximately proportional to the 
fifth power of the absolute temperature. This is 
because emission at the black-body maximum is 
proportional to the fifth power of the absolute tem¬ 
perature, and the emission at the wavelengths from 
8.5 p to 13.5 p lies near this maximum at ordinary 
temperatures. 

The approximation is expressed analytically as 
follows: 


13.5 n 


B = 


J XT dl = AT 5 = 1.9 X 10- 9 T 5 , (1) 


where B represents the far infrared brightness (in 
microwatts per square centimeter per steradian; 
B is the emission of a black body at T degrees 
absolute, integrated throughout the far infrared 
wavelength band 8.5 p to 13.5 p. 

The military value of infrared detectors for ob¬ 
serving the movements of ships offshore and the 
massing of military equipment and personnel in the 


225 











226 


FAR INFRARED DETECTING ELEMENTS 


field suggests itself immediately, and indeed a very 
considerable effort has been expended to develop 
heat-sensitive devices for purposes of this kind. 
Objects of the greatest interest from a military 
standpoint are generally at temperatures only 
slightly above those of the background and almost 
always below the boiling point of water. On the 
assumption that such objects as are of military 
interest may be regarded as black (i.e., objects 
which completely absorb and emit all wavelengths 
of radiation), a simple calculation shows that the 
preponderance of the radiation transferred to a 
black-body receiver lies in the wavelength interval 
5 p to 15 p. 

Throughout the spectrum from 0.8 p to the very 
long wavelengths, there exist regions of absorption 
due to molecules in the atmosphere. At these points 
the atmosphere is frequently almost opaque to 
infrared radiation, hence it is a fortunate circum¬ 
stance that there exists an atmospheric trans¬ 
mission window just in the region from 8.5 p to 
13.5 p. Figure 2 shows the observed transmission 
of the atmosphere for an optical path of 650 feet, 
with 0.25 cm of precipitable water vapor present. 



Figure 3. Solar energy transmitted by the Langley 
window. 

This transmission curve depicts the so-called “win¬ 
dow” in the atmosphere, discovered by Langley 
and first defined by Fowler’s measurements as re¬ 
produced in Figure 3. 

Figure 4 shows a graph of the transmission, as 


deduced from solar observations, using the 8.9-p 
residual ray band of quartz. The transmission is 
plotted against the amount of water vapor in the 
solar beam, and it may be presumed to be repre- 



CENTIMETER OF WATER AS DETERMINED BY <f> BAND ABSORPTION 

Figure 4. Centimeter of water as determined by 
3>-band absorption. 


sentative of the transmission of the whole 8.5-p to 
13.5-p band. The heat-sensitive devices must, there¬ 
fore, function for radiations in this wavelength 
interval. 

The developments have been of widely varying 
natures, but in every case the responsivity of the 
device has been related to the temperature change 
developed in the receiving element of the detector 
by the incident radiation. The great variety in the 
types of detectors which have been developed and 
built makes it very desirable that comparative 
studies be made to obtain as much information as 
possible about their relative merits, i.e., their re- 
sponsivities, response times, noise output, equiva¬ 
lent noise input, minimum detectable signal, etc. 
At the joint request of the Army and the Navy, 
Contract OEMsr-1168 was set up at the Ohio State 
University, as Project Control AN-6, under the 
auspices of Section 16.4 of NDRC, to make com¬ 
parative tests on the various American thermal 
detectors. In addition, tests on captured enemy 
thermal detectors were officially requested as Proj¬ 
ect Control SC-127. 

This chapter summarizes the work done on the 
development of far infrared detectors under Con¬ 
tracts OEMsr-126 and OEMsr-1147 at Massachu¬ 
setts Institute of Technology, under Contract 
OEMsr-60 at Harvard University, and under Con¬ 
tract OEMsr-636 at Bell Telephone Laboratories, 


irwiihiu pjpu 





TYPES OF DETECTORS 


227 


also the testing of these detectors under Contract 
OEMsr-1168 at the Ohio State University. The de¬ 
tectors developed by the above-named contractors 
are the Harris evaporated thermopile, the Strong 
nickel-strip bolometer, and the Bell Telephone 
Laboratories thermistor bolometer, respectively. 
Construction details and other data of a physical 
nature obtained from the final reports on these con¬ 
tracts, as well as performance data from OEMsr- 
1168, are included in subsequent sections. Brief 
paragraphs on the performance of other far infra¬ 
red detectors, developed outside Section 16.4 but 
tested under this section’s auspices, are also in¬ 
cluded in order that a comparison may be made of 
a wider variety of types. 

82 TYPES OF DETECTORS 

The several kinds of heat-sensitive units which 
have been developed fall essentially into three 
groups; namely, thermopile units, bolometer units, 
and the Golay cell. 

8,2,1 Thermopiles 

A thermocouple consists of two junctions between 
different kinds of metal, for example bismuth and 
antimony, and operates on the principle that when 
one junction suffers a temperature change relative 
to the other a thermoelectric voltage is developed. 
The magnitude of the electromotive force developed 
depends on the nature of the materials used, i.e., the 
thermoelectric powers, the dimensions and heat 
capacity of the element, and the blackness of the 
rectangular receivers affixed to the junctions. A 
thermopile is essentially a series of thermocouples, 
usually joined in series, but sometimes in parallel. 

Five thermocouples were submitted to Contract 
OEMsr-1168 for testing. Of these, the Harris evap¬ 
orated thermopile was the only Section 16.4 devel¬ 
opment; the others, obtained from commercial 
sources, were the Weyrich, Schwarz and two types 
of Eppley thermocouples. One of the latter was used 
with the Emerson detector and the other with the 
Farrand device. 

822 Bolometers 

A bolometer is made of a material which has a 
high temperature coefficient of resistance, either 
positive or negative. When the strip of material is 


exposed to heat radiation, its temperature will in¬ 
crease with an accompanying change in resistance. 

If the bolometer is incorporated in an electric net¬ 
work, the current flowing through it will be altered 
as the bolometer resistance varies. A measurement 
of the changes in currents in such an electric net¬ 
work can be utilized to determine the amount of 
thermal power falling on the bolometer strip. 

Eight different detecting units of the bolometer 
type were examined under Contract OEMsr-1168. 
Two of these, Bell Telephone Laboratories [BTL] 
and Radio Corporation of America [RCA] bolom¬ 
eters, were found to have negative temperature co- , 
efficients of resistance. (This type will hereafter be 
referred to as the thermistor bolometer.) Five of 
the remaining six units, the Strong nickel-strip, the 
Felix, the Polaroid, an Italian bolometer, and the 
German Donau Gerat are essentially of the metal- 
strip type. The sixth, the Andrews device, contains 
* a strip of columbium nitride which becomes super¬ 
conducting near the triple point of hydrogen. The 
BTL bolometer is distinctly a high-impedance de¬ 
vice; the RCA bolometer is of intermediate imped¬ 
ance, and the rest must be regarded as low- 
impedance instruments. 

82 3 The Golay Heat Cell 

The Golay heat cell, or photothermal unit, oper¬ 
ates on the principle that a pressure-volume ( p-v ) 
change occurs in a gas when its temperature is in¬ 
creased or decreased. The unit consists essentially 
of a metal chamber which contains the gas. The 
front of the chamber is closed by a membrane 
which acts as a receiving element, and the back is 
closed by a distensible mirror membrane. When 
radiant heat falls on the receiver, the temperature 
of the gas within rises, and the accompanying pres¬ 
sure increase causes the mirror to distend. Light 
from a small lamp is made parallel and passed 
through a grid of parallel lines and focused on the 
mirror by means of a meniscus lens placed near the 
mirror. The mirror in its undistended shape causes 
an image of the grid to be formed in the plane of 
the grid so that the grid-bar images and the real 
grid spaces coincide, thus permitting virtually no 
light to reach a photocell, the function of which is 
to transfer the p-v change into an electric impulse. 
When the mirror is distended by the increase in 
pressure, the image of the grid is focused in a 






228 


FAR INFRARED DETECTING ELEMENTS 


plane other than that of the grid so that light may 
now be passed by the combination to the photocell. 
This device provides a great deal of optical ampli¬ 
fication. The changes in photocell current may then 
be further amplified by electronic means. 

8 3 GENERAL CONSIDERATIONS 

831 Methods of Specifying the 

Responsivity of Detectors 

To specify the responsivity of heat detectors, a 
number of things must be taken into consideration. 
If a detector having a receiving area A (square 
centimeters) is in a radiation field of flux density 
F (watts per square centimeter), its responsivity 
may in general be given alternatively in terms of 
its output per unit of flux density (watts per 
square centimeter) or in terms of its output per 
unit of flux (watts). The latter may be obtained 
from the former by dividing by A. The various de¬ 
tectors operate on different principles and, there¬ 
fore, the output may be specified in different ways. 
In this chapter the responsivity defined in terms 
of volts per watt has been used exclusively. 

In the case of the thermopile, radiation falls on 
one set of junctions and an emf results, which can 
be measured by means of a galvanometer. The re¬ 
sponsivity might, therefore, be thought of as the 
ratio of the electromotive force generated to the 
temperature change developed between the hot and 
cold junctions. Since this change is directly related 
to the power incident on the receiver, the respon- 
sivity might also be regarded as the ratio of the 
thermal emf produced to the power (say in micro¬ 
watts) falling on the thermocouple. 

When radiation falls on a bolometer through 
which a current is flowing, the change in the bolom¬ 
eter’s resistance is recorded in the potential drop 
across its terminals. This may be measured by 
means of the voltmeter. If there is a ballast resist¬ 
ance in series with the bolometer, the responsivity 
can be specified in two ways: (1) in terms of emf 
per watt of incident power under specified operating 
conditions, as in the case of the thermopile; or (2) 
in terms of the fractional change in resistance 
(Aj R/R) per watt of incident power. These remarks 
apply both to the high- and low-impedance types 
of bolometers. 

The Golay cell, however, being a pressure- 
volume device, does not directly generate a voltage 


as the result of the absorption of heat radiation, 
and a different definition must be employed. To be 
sure, the unit has an auxiliary optical amplification 
system in conjunction with the heat cell which oper¬ 
ates a photocell circuit so that the incident power 
does establish a voltage, but this system is inci¬ 
dental to the heat cell. The responsivity of the heat 
cell alone could probably best be given in terms of 
the fractional pressure change, A p/p, per watt of 
incident power. 

On the basis of numbers representing the respon¬ 
sivity alone, difficulties would be encountered in 
comparing the performance of the detectors with 
each other. Because of this difficulty and because 
each type of unit has a different amount of noise 
associated with it, a different method of comparing 
them must be used. A method which measured the 
signal detected together with the noise was conse¬ 
quently chosen as the criterion. 

Noise Considerations 

The output from a thermal detector must, in 
general, be amplified in some manner before the 
response is sufficiently large to be recorded or indi¬ 
cated in some other manner. This may be accom¬ 
plished, for example, by the successive cascade of 
amplification stages which will allow a small signal 
to be built up to any. desired size. It might appear 
reasonable that the thermal detector which will 
produce a usable output with the smallest amount 
of amplification would be the best detector to use. 
This, however, is not necessarily true, since a device 
which may be regarded as an extremely sensitive 
detector, and, therefore, may operate with small 
amplification, may have an inherent noise which is 
more than proportionately large. An inherently very 
quiet, but insensitive detector with a high-gain 
amplifier may be able to detect smaller heat signals 
than an inherently noisy, but highly sensitive de¬ 
tector with a low-gain amplifier. The excellence of 
a detector is its ability to detect a signal and to 
distinguish it from the inherent noise of the system. 

The noise at the output-indicating mechanism 
where the signal response is viewed will consist of 
the noise originating in the thermal detector plus 
noises originating in the amplifier. In the region 
viewed or scanned by the detector there may, of 
course, exist thermal fluctuations in the back¬ 
ground in addition to those which it is desired to 




GENERAL CONSIDERATIONS 


/ 

- 


229 


detect. Target fluctuations of this sort which tend 
to obscure the signal are not considered in this 
chapter. The noise originating in the detector and 
in the amplifier cannot be completely eliminated by 
any design of the system. These limiting noises are 
inherent in the nature of the detecting system. 

Limits of Usefulness Because of Noise 

The nature of the inherent noises in a detecting 
element cannot here be discussed in detail. They 
may, however, be visualized in somewhat the fol¬ 
lowing manner. Consider a mirror suspended by a 
fiber in a medium of gas particles. Assume, more¬ 
over, that a light beam is reflected by the mirror 
and is focused at a point so as to produce a trace 
on a moving photographic film. If the mirror were 
clamped rigidly, the trace would be a straight 
(noiseless) line on the photographic film. 

The gas particles are, however, constantly exe¬ 
cuting random motions (Brownian motions), the 
magnitudes of which are related to the absolute 
temperature of the gas. This random motion is 
communicated to the suspended mirror by collisions 
of the particles with the mirror. The mirror will, 
therefore, suffer random fluctuations from its zero 
position and the trace on the photographic film will 
no longer be a straight line but will contain zigzag 
deviations. The lack of definition produced in this 
manner is indicative of the Brownian noise present. 

It might appear in the foregoing naive example 
that the effect could be minimized or even reduced 
to zero by pumping away the gas. It is, of course, 
clear on second thought that the random motions 
of the gas particles are not the only random mo¬ 
tions present. There will also be the random motions 
of the atoms in the fiber, random motions of the 
atoms where the fiber is supported, and so forth. 
This is consistent with the analysis, as it rests 
entirely on statistical laws and does not depend on 
the mechanism of the energy fluctuations. 

In electrical phenomena it is the velocities of the 
electrons themselves which fluctuate randomly. In 
a vacuum tube these random motions give rise to 
the shot effect (statistical fluctuations in the rate 
of emission of electrons), and in an electrical circuit 
they manifest themselves as the Johnson noise 
(fluctuations in the voltage across an impedance 
element). 

In addition to the Johnson noise originating* in 
the thermal detecting elements, there may be other 


noises such as microphonic or current noises. Dur¬ 
ing tests made under Contract OEMsr-1168 very 
little microphonic difficulty was encountered with 
any of the detecting elements studied. This was not 
entirely true of these devices when used in the 
field. Current noise is a voltage fluctuation across 
a resistor, the existence and magnitude of which 
depends upon the current passing through the re¬ 
sistor. When the current passing through the resistor 
is zero, the current noise is zero. Current noise is 
predominantly a low-frequency phenomenon and 
the noise voltage components decrease rapidly with 
increasing frequency. Since a bolometer is a de¬ 
tecting device which may carry a considerable cur¬ 
rent through it when operating, current noise may 
become significant. In tests made on certain devices, 
current noise was quite evident. 

The Johnson noise is the form of thermal agita¬ 
tion which is of principal interest, since it sets a 
lower limit on the least detectable signal. The 
Johnson noise measured across a resistor will de¬ 
pend upon the temperature of the resistor, the mag¬ 
nitude of the resistance, and the width of the 
frequency band in which the noise is measured. 
The mean square noise voltage due to thermal 
agitation, v 2 , integrated over all components be¬ 
tween the frequencies /i and / 2 , may be shown to be 

v 2 = 4/cT7? (/ 2 — ji ), (2) 

k being the Boltzmann constant, T the absolute tem¬ 
perature, and R the resistance. At a temperature 
of 290 K, (2) becomes 

v 2 = 1.59 X 10“ 20 tf (/ 2 - /i) volt 2 . (3) 

After amplification by an amplifier of gain G fj the 
voltage available for measurement will be 


v 2 = 1.59 X 10~ 20 R 



Gf 2 df 


= 1.59 X 10 -mRWGm, (4) 

where A/ denotes the effective noise bandwidth, 
G m the maximum value of G f , and R is presumed 
to be a constant over the frequency bands for which 
G f is large. A measuring system of conventional 
design will contain a band-pass amplifier over a 
single band of frequencies with a maximum gain 
G m within this band and a zero gain outside. The 
effective noise bandwidth may then be evaluated 



230 


FAR INFRARED DETECTING ELEMENTS 


from a knowledge of the experimental gain data 
and the relation 



The effective noise bandwidth for the amplifiers 
used in the experiments on which this report is 
based was obtained in this manner. 

Estimation of Expected Noise 

It is possible that the lower noise limit set by 
the Johnson noise of the detecting element may 
never be reached because of noise which may orig¬ 
inate in the amplifying system. These amplifier 
noises which add to the noise of the detecting ele¬ 
ment itself may be of the following kinds. 

1. Johnson noise and current noise in the ampli¬ 
fier parts, particularly in those parts associated 
with the input bridge circuit. 

2. Shot noise fluctuations in the space-charge- 
limited vacuum-tube currents, caused by the ran¬ 
dom emission of the electrons. 

3. The flicker effect caused by changes in the 
surface conditions of the emitter (confined to low- 
audio and sub-audio frequencies). 

4. Microphonics associated with mechanical vi¬ 
bration of the charged units, tube electrodes, and 
faulty contacts. 

It has been the experience of those working under 
Contract OEMsr-1168 that the total output noise 
was invariably larger than the Johnson noise to be 
expected from the input circuit. For this reason an 
extrapolation from the amplifier noise to the ex¬ 
pected Johnson noise has been made in all cases in 
order to evaluate the performance of the detector. 
In this manner, the lowest limit of usefulness of the 
device has been estimated. In this report there will 
be found, therefore, not only the best results which 
it was possible to obtain under Contract OEMsr- 
1168, 2 but also the best results which might be 
expected if a perfect amplifier were used with the 
detector, and if the only noise originating in the 
detector were Johnson noise. 

The noise in the output can all be reduced by 
limiting the bandwidth of response of the output 
indicating mechanism. When sufficient time is avail¬ 
able for the detection of a signal, a slowly respond¬ 
ing indicator may be used. This use implies that the 
effective frequency pass band A/ becomes narrower, 


and, in effect, averages out some of the noise. Thus, 
for example, an observer watching the fluctuating 
needle of an instrument for a period of time would 
automatically apply an averaging process and 
thereby, through elimination of some of the com¬ 
ponent frequencies, reduce the effect of the noise in 
obscuring the signal. If, on the other hand, only a 
short interval is available for the detection and 
perception of a signal, a rapidly responding instru¬ 
ment must be used, and as a result the effective 
frequency pass band is widened. A more rapid 
method of detection is inherently accompanied by 
a larger amount of noise than is a slow method of 
detection. 

83 3 The Equivalent Noise Input 

It has been indicated earlier that the real merit 
of a detector is reflected in its ability to distinguish 
a signal from the noise. This quality has been made 
the basis for a comparison among the various de¬ 
tectors studied. The actual quantity which has been 
determined in each case is known as the equivalent 
noise input [ENI] and represents the amount of 
signal input necessary to produce an output equiva¬ 
lent to the noise. A more explicit definition of the 
ENI follows. 

ENI = limit noise voltage output) 

f signal in-»o (rms signal voltage out) 

1 signal out-*o (signal input) 

where rms is used as an abbreviation for root mean 
square. The denominator is the limiting value of 
the responsivity for very small signals. When the 
amplification is linear over the entire range of noise 
and signal levels, the limiting process is unneces¬ 
sary. The input signal has been measured in peak- 
to-peak microwatts for all values in this report. 

The ENI was obtained in actual tests by plotting 
the output from the amplifier against the heat sig¬ 
nal applied to the thermal element. Whenever the 
output of the amplifier was proportional to the 
input, the ENI could be obtained by extrapolating 
the curve to zero and noting for what value of the 
input the ordinate has the same value as the output 
noise, the latter being read when a shutter was inter¬ 
posed between the detector and the heat source. 
This is illustrated in Figure 5. 

It has already been pointed out that the output 
noise observed was greater than the Johnson noise 








GENERAL CONSIDERATIONS 




231 


which one might expect from the thermal element 
alone. The ENI becomes, in reality, an indication 
of the sensitivity, since the more sensitive units 
will give the smallest ENI for a given output noise 
level. The reasons for this are believed to be inti¬ 
mately related to the fact that the amplifier must 
be tuned to frequencies in the low-audio range in 



HEAT INPUT IN MICROWATTS 

Figure 5. Heat test on Felix bolometer with black 
body and tuned amplifier. 

order that the thermal elements themselves may 
operate effectively. At these frequencies the flicker 
noise in the first stage of amplification may be 
sufficient to exceed the Johnson noise. For this 
reason a minimum equivalent noise input [MENI] 
has consistently been determined also. The MENI 
is the heat signal computed to be necessary to pro¬ 
duce an rms voltage output equal to the rms John¬ 
son noise voltage of the thermal detector alone. 
This would be the ENI expected if a noiseless 
amplifier and bridge circuit could be built and if 
the thermal detector had no other noises than 
Johnson noise. 

The units in which the ENI and MENI are 
given are the same as those in which the input 
signal is measured, microwatts, ergs per second, 


etc. In the determination of these quantities the 
peak-to-peak value of the square-wave-heat power 
incident upon the thermal element has consistently 
been employed. This represents the magnitude of 
the d-c power available for detection before it is 
chopped into a square wave. 

The ENI and the MENI will both depend, as has 
already been suggested, on the pass band of the 
amplifier. In general, the same tuned amplifier was 
used on all the thermal detectors studied, except 
the BTL bolometer, so that the pass-band width 
has been the same in virtually all the comparative 
tests made under Contract OEMsr-1168. 

83 4 The Minimum Detectable Signal 

Another indication of the merit of a thermal de¬ 
tector is the minimum detectable signal [MDS]. 
This been been regarded as an estimate of the mag¬ 
nitude of a signal required to produce a response 
on an Esterline-Angus recording milliammeter 
which can just be definitely seen above the noise. 
This quantity was evaluated by recording the out¬ 
put from the tuned amplifier on the Esterline-Angus 
meter while signals were impressed on the thermal 
elements at intermittent intervals, a record of the 

100 
90 
80 
70 
60 
50 
40 
30 
20 
10 
0 

Figure 6. Determination of minimum detectable sig¬ 
nal for Strong bolometer at 27.6 c tuned amplifier. 

magnitude of the signal being made each time. From 
these records an estimate was made of the least 
signal which could definitely be distinguished from 
the noise with a signal exposure time of about 
30 seconds. This is illustrated in Figures 6 to 11 
inclusive. 

It is evident that neither the ENI nor the MDS 
can be accurately measured, since they are both 





















232 


FAR INFRARED DETECTING ELEMENTS 


obtained from measurements relative to a noise 
background. They must, therefore, be as difficult to 
define and determine accurately as the noise itself. 
While the absolute value of the noise is not readily 



Figure 7. Determination of minimum detectable sig¬ 
nal for Felix bolometer No. 19 at 27.6 c tuned am¬ 
plifier. 


estimated or metered when a narrow pass band 
system is used, measurements of the above type do 
allow a comparison of the relative merit, at least in 
certain specific characteristics, of the several units 
studied. 



Figure 8. Determination of minimum detectable sig¬ 
nal for BTL Becker bolometer No. 19 at 15 c. 


Under Contract OEMsr-1168, a considerable 
number of devices were found to have ENI meas¬ 
urements which differed from each other by not 
more than a factor of two. When this is so it be¬ 
comes quite difficult to attain a valid judgment of 



Figure 9. Determination of minimum detectable sig¬ 
nal for RCA bolometer No. AX at 14.6 c. 



Figure 10. Determination of minimum detectable 
signal for superconducting bolometer No. 4 at 27.6 c. 


the merit of a device on the basis of noise measure¬ 
ments alone. In such instances, other factors, such 
as frequency response, compactness, and ruggedness, 
must be regarded as the differentiating character¬ 
istics. 

























































































































































































































































GENERAL CONSIDERATIONS 


233 



Figure 11. Determination of minimum detectable 
signal for superconducting bolometer No. 1 at 27.6 c. 


fall on the thermopile long enough to establish 
equilibrium, the Voltage generated per watt is not 
given by $ d _ c , but by the smaller number 

Sa - c < t > W , (8) 

where 

_response at frequency / 

^ response at zero frequency 

obtained from the frequency-response curve, and 
W = waveform factor, to take account of the man¬ 
ner in which the heat power is interrupted. The 
voltage at the output of an amplifier of gain G f 
per watt of heat power input on the thermopile 
could, therefore, be expressed by the following 
relation: 

=Gf(Sa- c )<f>W volts per watt. ( 10 ) 

dH 


8 . 3.5 O u tput of Thermopiles Predicted 
from the d-c Responsivity 

When radiation of an amount dH (watts), effec¬ 
tive in causing a temperature rise, falls on a thermal 
junction for a time long enough to produce an equi¬ 
librium between the heat power absorbed and the 
heat power lost, a thermal emf dV is generated. 
If there are n junctions in series, the voltage devel¬ 
oped is ndV. The responsivity, S d _ c , in volts per 
watt of uninterrupted incident radiation, may then 
be expressed as 


S d -c — w-ttj volts per watt. 
dH 


(7) 


This expression can be made equivalent to equation 
(3) of reference 3, 


F d _ c _ nm 

where F d _ c = the voltage response to steady-state 
radiation, 

n=the number of hot junctions, 

m— the volts per degree for each junc¬ 
tion, 

G — the heat power absorbed by the ther¬ 
mopile in watts per square centi¬ 
meter, 

L = the heat power lost by the thermopile 
per degree rise in temperature in 
watts per square centimeter if each 
term is divided by the area. 

If, however, the radiation is interrupted at a 
definite frequency so that the radiation does not 


8 3 6 Output of Bolometers Predicted from 
the Static Characteristics 

When radiation of an amount dH is permitted to 
fall on a bolometer strip of resistance R b carrying 
a constant current 7 & , the strip suffers a temperature 
change AT and, consequently, a resistance change 
A R b . As a result, the potential difference across the 
bolometer will change by an amount which is 
7&A7£&. 

Frequency Factor <f> 

If the radiation is interrupted periodically so 
that the time of illumination is short in comparison 
with that required for the establishment of tem¬ 
perature equilibrium, the voltage change will be 
somewhat smaller than for uninterrupted illumina¬ 
tion. The resistance fluctuations, and hence the 
voltage fluctuations, depend critically on the fre¬ 
quency of interruption of the illumination. This 
effect is clearly shown by the frequency-response 
curves, in which the response is seen to decrease as 
the frequency rises from the zero value. The zero- 
frequency voltage change across a bolometer for a 
heat power input of one watt could, therefore, be 
expressed by I b (dR b /dH ). 

To obtain the voltage change at any other fre¬ 
quency, a multiplicative factor, which is the ratio 
of the response at the particular frequency to the 
response at zero frequency, must be applied. This 
factor may be obtained from the frequency-response 
curve. 









































































234 


FAR INFRARED DETECTING ELEMENTS 


Waveform Factor W 


In addition, the waveform factor W must be 
introduced, the value of which depends on the 
manner in which the heat power is applied and 
the output is measured. If the heat power input on 
the bolometer is a square wave measured peak to 
peak, and the bolometer output is fed into a tuned 
amplifier which passes only the fundamental com¬ 
ponent and this output is measured in rms volts, 
then W is 0.45. If the heat input is a sinusoidal 
wave measured peak to peak and the output is 
measured in rms volts, W is 0.35. 

General Response Equation 

When this voltage change is fed into an amplifier, 
the gain of which is G f at the frequency of opera¬ 
tion, the voltage output per watt of heat power 
input may, in general, be stated as follows: 


dE out 

dH in 


= G,I b 


dH , 


-<t>W. 


(ID 


Where direct measurement of ( dR b /dH ) is not pos¬ 
sible, variations of this relation may be used to 
predict the voltage output. Thus, from the amplifier 
characteristics and the static characteristics, the 
bolometer output for any frequency may be com¬ 
puted. For the bolometers tested, the outputs have 
been computed and compared with the measured 
output. In general, the agreement has been good. 


Circuit Dependence and Bridge Factor F 

In a bolometer circuit the bolometer current I b 
may not be a constant when the bolometer resist¬ 
ance is varied. The bolometer voltage change pro¬ 
duced by a change in the bolometer resistance is not 
just IbA R b , as presumed in the foregoing discussion, 
but A (/&#&). The change in bolometer voltage de¬ 
pends upon A/ b as well as A R b and will, therefore, 
depend upon the circuit in which the bolometer is 
placed. The expression G f I b ^F h , however, is the 
output voltage from the bolometer network, which 
may or may not contain transformers and vacuum 
tubes, if G f is given the proper value. This value of 
G f is the change in the output voltage in the net¬ 
work produced by the introduction of a 1-volt, zero- 
impedance source in series with the bolometer, all 
other circuit elements remaining the same. 

In the case of the thermistor bolometers, which 
do not use a transformer to couple to the amplifier, 


the bridge factor of the bolometer network has been 
used. This bridge factor F is the G f defined above 
but is applied only to the network which furnishes 
electrical power to the bolometer and from which 
the signal is taken for amplification. 


8 3 7 Frequency Response of the 
Detectors 

When modulated heat signals are received by 
infrared detectors, the response of the detector 
varies in some manner depending on the frequency 
of modulation of the signal. For bolometers, this 
problem has been studied in great detail, by a num¬ 
ber of authors, 4 with respect to the various factors 
affecting the response, such as the thermal capacity 
of the element and its heat dissipation through radi¬ 
ation and conduction. In general, the response of 
the Golay cell fails at low as well as at high fre¬ 
quencies, and for this unit there exists a frequency 
of maximum response. Some units have the charac¬ 
teristic that when they are illuminated by radiant 
energy, the heating curve approaches a steady- 
state value exponentially. Similarly, when the radi¬ 
ation is shut off, the cooling curve falls off exponen¬ 
tially from the steady-state value to zero. Such heat¬ 
ing and cooling curves are shown in Figure 35. The 
frequency behavior of such units is characterized 
by a time constant t, which is the time required by 
the unit to reach a value on the heating curve of 
(1 — 1/e) times its steady-state value (or to fall 
to a value in the cooling curve of (1/e) times the 
steady-state value). For such units the voltage re¬ 
sponse to sinusoidally interrupted radiation may be 
given by the relation 

F ou *= (i -LsL— t (12) 

where F ou t is the response in volts, V zf is the re¬ 
sponse at zero frequency, co = 2jt/, and r is the time 
constant in seconds. The time constant t may also 
be given as the ratio of the effective heat capacity 
H of the bolometer to the total heat dissipation 
constant C, x = H/C. At low frequencies of modu¬ 
lation, co 2 t 2 is small compared with unity and the 
response is independent of the frequency. At high 
frequencies the co 2 t 2 term is large compared with 
unity and the response is proportional to 1//. On a 
logarithmic plot the response approaches these two 
situations asymptotically, and at the frequency / 0 , 






GENERAL CONSIDERATIONS 


I 

- 


235 


where these asymptotes intersect, the response is 
down 3 db from the zero-frequency asymptote. At 
this frequency, oot =1 and the time constant 
x = l/(2jt/ 0 ). 

If t is small, then the response is not affected by 
as low frequencies as it is if t is large. For a given 
element, if the heat dissipation can be increased, 
the time constant can be reduced with the same 
effect as is achieved by reducing thermal capacity. 
The result of this is to make a flatter response over 
a larger frequency interval. Various devices have 
been used, such as the introduction of gases and 
cooling plates to increase the heat dissipation. These 
devices in some cases alter the shape of the response 
curve so that the unit no longer behaves as if it had 
a true time constant and the high-frequency be¬ 
havior may approach 1/V7 rather than 1//. 

It was the practice in the tests made under Con¬ 
tract OEMsr-1168 to make a frequency-response 
curve using a sinusoidal heat input of constant 
maximum amplitude and to measure the output at 
enough frequencies to determine both the low- and 
high-frequency behavior. The output in decibels 
was plotted against log frequency. The decibel num¬ 
bers given on the frequency response are not the 
same for any two curves and are not related to the 
actual responsivity of the element. From the experi¬ 
mental response curve, it is then possible to deter¬ 
mine whether the unit has a true time constant. If 
the response curve does not have the proper form, 
no time constant is reported, but the response can 
be read from the curve. In the case where no time 
constant exists, a substantial error might be made 
if an effective time constant were to be stated. The 
performance of some of the bolometers tested can 
be approximated by sensitivity and time constants 
over limited frequency ranges. The use of such time- 
constant data leads to erroneous results if applied 
outside the proper frequency range and may be 
radically in error if extrapolated to zero frequency 
(see Section 8.7.3). 

The various detectors tested were designed to 
operate under widely different conditions. The BTL 
insulator-backed bolometer and the Schwarz and 
Harris thermopiles were designed to operate at 
atmospheric pressure, while the Strong and Felix 
bolometers were made to operate at reduced pres¬ 
sures in hydrogen gas. Some of the BTL bolometers 
were backed with various materials such as quartz, 
glass, rock salt, and plastic, while others were un¬ 


backed. The fact that few of the tested detectors 
had simple time constants was doubtlessly related 
in some manner to these design factors. 

Spectral Characteristics of the 
Detectors 

As the infrared detectors sent to Ohio State 
University for testing were of widely different de¬ 
sign and materials, it was considered of interest and 
importance to investigate their response to radiation 
of different wavelengths. The detecting elements of 
the receivers were, in general, blacked with some 
material such as platinum, gold, or aluminum black. 
The detectors designed to operate at reduced pres¬ 
sure were equipped with a window through which 
the radiation must pass. Those operating at atmos¬ 
pheric pressure also required windows to eliminate 
acoustical pressure effects and moisture condensa¬ 
tion on the elements. 

The window materials used were silver chloride 
on the Strong, Felix, and RCA bolometers; rock salt 
on the Italian bolometer, the Harris thermopile, the 
Golay heat cell, and the Andrews cryostat; potas¬ 
sium bromide on the Weyrich thermocouple; gil- 
sonite or silver sulfide coated rock salt and silver 
chloride on the BTL thermistors; fluorite on the 
Schwarz thermocouples; and thallous bromide- 
thallous iodide mixture on the Donau Gerat bolom¬ 
eter. Both blacking and window material affect the 
response as a function of the wavelength of the 
radiation. Because of the necessity for the use of a 
window, it was not possible to isolate window effect 
and blacking effect absolutely, though a fairly good 
guess could be made. In the case of the Strong 
bolometer, the coated silver chloride window was 
removed and a polished rock salt window was put 
on instead, with the result that the response was 
increased by a factor of two. In the case of the 
Felix unit No. 2A, the silver chloride window was 
exposed to sunlight with the effect that the 1-p to 
4-p response was appreciably lessened without re¬ 
ducing the response throughout the remainder of 
the region by more than about 20 per cent. 

Definition and Measurement of Absorptivity 

The spectral response from 1 p to 14 p for each 
device was obtained by using the detector to receive 
radiation from a Nernst glower dispersed through a 
small Hilger rock salt prism spectrometer. The re- 


Ke gjEB 




236 


FAR INFRARED DETECTING ELEMENTS 


sponse of each detector was compared with the 
response of a Coblentz thermopile throughout the 
same region, and the ratio of the detector response 
to the Coblentz response was plotted against wave¬ 
length. The Coblentz thermopile was considered to 
be uniformly black over the entire range. 

In the case of the thermopiles, the ratio of the 
responses was set equal to unity at 2.0 p, while for 
the bolometers this ratio was made to equal a 
quantity called the absorptivity , a 2 p, defined as the 
ratio of the resistance change per watt of heat power 
input to the resistance change per watt of electrical 
power input (dR/dH) / (dR/dP ). 

The quantity a * obviously depends on the wave¬ 
length of the radiation, because it really is a meas¬ 
ure of the fraction of the incident heat power which 
is effective in causing a resistance change, and this 
fraction depends on characteristics varying with 
wavelength, such as the blackness of the receiver 
and the transmission of the window material. 

The absorptivity was determined by the follow¬ 
ing method: The bolometer resistance was meas¬ 
ured with a Wheatstone bridge, the voltage supply 
being varied. Thus, it became possible to plot a 
graph of R b versus P (electrical power expended in 
the bolometer), as shown, for example, in Figure 12, 
from which dR/dP could be obtained. The term 
dR/dH was obtained by one of two methods. 

Static Method. The term dR/dH was obtained 
by direct measurement of A R for a certain amount 
of heat power A H incident on the bolometer as 
follows: Radiation from a Nernst glower, which is 
principally 2-p radiation, was focused directly on 
the bolometer strip, while the resulting A R was 
measured by a potentiometer method. A H was 
measured at the same time by means of the cali¬ 
brated Coblentz thermopile. The ratio (dR/dH) ^ 
could then be calculated and the ratio of (dR/dH) 
to dR/dP, obtained from the R versus P curve, is 
the absorptivity a x for 1 principally 2 \x. 

Dynamic Method. The term dR/dH was also 
obtained by a method which utilizes equation (11). 
Radiation from the Nernst glower was again focused 
directly on the bolometer strip and interrupted 
sinusoidally at 20 cycles. The voltage developed by 
the bolometer was fed into the primary of a trans¬ 
former by means of a bridge circuit and the voltage 
at the secondary was measured by a Ballantine 
electronic voltmeter. I b , the current through the 
bolometer, was measured with a calibrated volt¬ 


meter and shunt. A H was measured peak to peak 
by the calibrated Coblentz thermopile method. 
With the frequency response of the device available, 
all quantities in equation (11) are known with the 
exception of dR/dH, which can then be calculated. 
The transformer gain at 20 cycles was measured 
using an input circuit equivalent to the bolometer 
input. 



ELECTRIC POWER IN WATTS 

Figure 12. Static curves for Strong bolometer. 

81 TEST EQUIPMENT AND METHODS 
DEVELOPED UNDER CONTRACT OEMsr-1168 

Those working under Contract OEMsr-1168 were 
concerned with the comparative testing of various 
infrared detectors. Because a large part of the data 
on the detectors was obtained under that contract, 
the test methods developed under it are discussed 
here. 

To facilitate the accurate measurement of small 
amounts of heat power, certain pieces of test equip¬ 
ment were constructed and certain more or less 
standard testing procedures were developed. The 
equipment w^as designed to be adaptable to a variety 
of detecting devices, so that a transfer from one 
device to another could readily be made. The equip¬ 
ment constructed and used in nearly all the tests, 
as well as the methods developed, will be discussed 
in the following paragraphs. Special testing equip- 







DEVELOPMENTS UNDER CONTRACT OEMsr-1168 


/ 

I 


237 


ment equivalent to that which is discussed in this 
chapter was developed by BTL and is discussed in 
memoranda issued by that laboratory. 5 ’ 0 

Heat Sources 

As a radiation standard, a calibrated incandes¬ 
cent tungsten lamp was obtained from the National 
Bureau of Standards. Other sources used in the tests 
were calibrated against this lamp. The calibrating 
equipment comprised a Coblentz thermopile and a 
high-sensitivity Leeds and Northrup galvanometer. 
The thermopile, galvanometer, and a small resist¬ 
ance were connected in series. A thermal emf, gen¬ 
erated by the known radiation flux of the standard 
lamp, caused the galvanometer to deflect. A known 
potential difference was introduced into the same 
circuit by passing a current through the small re¬ 
sistance with no radiation on the thermopile, and 
the galvanometer deflection was again read. From 
these deflections, and from the known radiation flux 
density and impressed emf, the responsivity of the 
Coblentz thermopile in volts generated per watt per 
square centimeter of radiation flux density was de¬ 
termined. Radiation flux densities from other sources 
than the standard lamp could then be measured 
rather easily. 

The principal secondary radiation source used in 
most of the tests was a small low-temperature black 
body. This was a small brass cylinder about 1 inch 
in diameter and about 4 inches in length, with a 
conical cavity turned in the front end. The cavity 
was coated with lamp black. An electric heating 
coil, operated from a 6.3-volt transformer, was in¬ 
side the cylinder and raised the temperature to 
about 115 C. 

The black body was mounted behind an insulat¬ 
ing board with a circular hole. A metal shutter, 
backed with insulating board, was inserted between 
the black body and the hole. The Coblentz thermo¬ 
pile, situated 30 cm in front of the .emitting hole, 
viewed the hole and insulating board background. 
The temperature difference seen was, therefore, that 
between shutter and black body. This temperature 
difference was obtained by means of a pair of 
copper-constantan thermal junctions, the hot one 
imbedded in the black body and the cold one 
attached to the metal shutter which, because of the 
backing, remained at room temperature. The range 
of temperature differences was about 0 to 90 C. 

The Coblentz thermopile was used to determine 


the flux density in microwatts per square centi¬ 
meter for the various temperature differences as the 
black body cooled down to room temperature. A 
plot of these points agrees very well with the plot 
of flux densities versus temperature differences as 
obtained from Stefan’s law, 

F — 1.69 X 10 _12 A—— 0 ^ 0 watts per sq cm, 
r~ 

where F is flux density in watts per square centi¬ 
meter, A is the area of the shield hole, T± and T 0 
are the absolute temperatures of source and receiver, 
respectively, and r is the distance between the shield 
and the receiver. 

In the various tests performed on the heat detec¬ 
tors, one of a variety of shields was affixed to the 
emitting hole to limit its size and shape. To provide 
a square-wave heat input, a rotating sector wheel 
was located between the black body and the hole, 
and a rectangular opening was used on the emitting 
hole, the hole width being much smaller than each 
of the sector arcs. For a sinusoidal heat input, a 
wheel with a sinusoidal periphery was turned in 
front of a slit opening. Another method was to use 
a sector wheel and a hole cut in the shape of one half 
of a sine wave, the base of which is just the width of 
the sector in the wheel. 

The temperature of the black body was varied 
between 20 and about 115 C, so that the radiation 
transferred to the receiver was chiefly in the 5-p to 
14-p spectral region. In some of the tests performed, 
e.g., the frequency response and spectral response, it 
was more convenient to use a hotter source or one for 
which the radiation was peaked in the near infrared 
so that it contained a considerable amount of visible 
light. The Nernst glower was a natural choice be¬ 
cause of its size and ease of operation. It is a rather 
narrow cylindrical source, constructed of a material 
with a negative temperature coefficient of resistance 
and made to operate in air. Its radiation peak is nor¬ 
mally at about 2 p and it is a fairly close approxima¬ 
tion to a black body in this region. It deviates appre¬ 
ciably from black-body characteristics beyond 4 p. 
This source is particularly convenient for frequency- 
response measurements because a visible small image 
of the source can easily be focused on the detecting 
element by means of a mirror. Because it is a narrow 
elongated source, a sinusoidal heat signal can rather 
easily be obtained by having a sinusoidal wheel in¬ 
terrupt the radiation from it. The flux density was 




238 


FAR INFRARED DETECTING ELEMENTS 


obtained as usual by the Coblentz thermopile. In the 
measurements of the spectral response of the de¬ 
tectors with the rock-salt prism spectrometer, the 
Nernst glower was a convenient source of infrared 
radiation because of its slitlike shape and because it 
radiates a great deal of heat energy. 

Amplifiers 

The amplifiers used in the tests were of two Jypes, 
sharply tuned and wide band. For the frequency- 
response measurements of the various devices, a wide 
band-pass amplifier was experimentally determined 
so that the frequency-response curves for the various 
devices could be obtained. The low-frequency re¬ 
sponse was tested by means of an a-c voltage output 
from a photocell which received light of sinusoidally 
varying intensity. The frequency of the light varia¬ 
tion was controlled by a sector wheel arrangement. 
Some heat-response measurements were also made 
with this amplifier. 

Most of the heat measurements, from which the 
ENI were determined, were made with tuned ampli¬ 
fiers of which two were available. One of these, for 
15 cycles, was supplied by BTL under Contract 
OEMsr-636 for use with thermistor bolometers de¬ 
signed by that laboratory and is described in one 
memorandum. 7 For the low-impedance bolometer, a 
tuned amplifier modified from specifications sup¬ 
plied by Harvard University Contract OEMsr-60 
was used. The tuned output stages were sepa¬ 
rated from the input stage so that several types 
of input stages can be used with these or with the 
wide-band amplifier. This amplifier employs a Thor- 
darson T-48094 input transformer which can be 
matched with the various units. 

Several sets of twin-T circuits were built to be 
plugged into the amplifier, making operation possible 
at 14.6, 27.6, 47, 80, and 140 cycles. These circuits 
were quite sharply tuned, the noise band-pass being 
about 3 cycles for all frequencies except 14.6, for 
which it was 0.6 cycle. The twin-T networks had to 
be carefully adjusted for proper operation. 

For each operation the gain was measured with a 
circuit equivalent to that of the bolometer. A 
Hewlett-Packard oscillator furnished the signal for 
these measurements and both input and output volt¬ 
ages were measured with Ballantine electronic volt¬ 
meters. 

The heat-response measurements were made with 
a square-wave heat input measured peak to peak. 


The tuned amplifiers pass only the fundamental 
sinusoidal component of the square-wave input, and 
the output voltmeter reads the rms value of this 
component. 

Frequency Response 

To determine the frequency response of the vari¬ 
ous detectors a more or less standard procedure was 
followed, although it was necessary to vary the pro¬ 
cedure slightly from case to case. The detector was 
irradiated by a sinusoidal heat input, the wheel 
interrupting the radiation beam being driven by a 
Cenco variable-speed rotator. The wheel used has 
already been described under “Heat Sources,” Sec¬ 
tion 8.4. By this method frequencies from 1 to 300 
cycles were produced. 

When frequencies greater than 300 cycles were 
required, the black-body source was used with an 
opening in the shape of one half of a sine wave. A 
wheel with many sectors was used to chop the radia¬ 
tion, the width of the sector being the same as the 
base of the sine opening. The wheel was driven by a 
high-speed motor, and frequencies up to 2,000 cycles 
were thereby made available. 

The output of the detector was amplified through 
the wide band-pass amplifier the frequency response 
of which was known over the entire range. This is 
described in the preceding section, “Amplifiers.” 
During the frequency-response measurements on 
any detector the heat input was maintained con¬ 
stant. 

If the frequency-response curve indicated that the 
thermal detector had a time constant, the parallelo¬ 
gram method was also used in certain cases. In this 
method, radiation chopped into square waves is 
permitted to fall on the detector. The detector out¬ 
put voltage is amplified without distortion of the 
wave shape and applied to one pair of oscilloscope 
deflection plates. The amplified output is also differ¬ 
entiated electrically and applied to the other pair 
of plates. The pattern which appears on the screen 
is a parallelogram. The slope of the proper sides of 
the parallelogram is proportional to the time con¬ 
stant of the thermal detector. A proportionality fac¬ 
tor may readily be adjusted by setting the relative 
gains of the vertical and horizontal amplifiers on 
the oscilloscope. Even if the thermal detector does 
not have a time constant, the oscilloscope figure may 
still look enough like a parallelogram to give the 
impression that a time constant exists. The method 







THERMOPILES DEVELOPED IN SECTION 16.4 


239 


is, therefore, merely supplemental to the frequency- 
response curve method. 

Spectral Response 

As the detectors tested were of various degrees of 
blackness and were fitted with windows of various 
materials, such as rock salt, fluorite, and silver 
chloride (coated and uncoated) in various states of 
transparency, it was considered of value to deter¬ 
mine their response to radiation of various wave¬ 
lengths. This response was obtained from 1 p to 14 p 
for the detectors and compared with the response 
of a Coblentz thermopile over the same range 
under similar conditions. 

For these measurements, a small Hilger rock-salt 
prism spectrometer of Wadsworth design was used 
in conjunction with a Nernst glower. A sector wheel 
was used between the source and the first spectrom¬ 
eter slit to interrupt the radiation. The energy from 
the exit slit of the spectrometer was focused on the 
detecting element by means of a short-focus mirror. 
The spectrometer was calibrated to read wavelengths 
in microns after being adjusted to some known spec¬ 
tral line. The absorption by C0 2 at 4.25 p was used 
as the standardizing line, and the absorptions by 
water vapor at 2.7 p and 6.2 p were used as further 
checks on the calibration. The slit widths used for 
the detectors were the same as those used for the 
Coblentz thermopile, so that a fair comparison could 
be made between the various detectors with the 
Coblentz as a standard. 

The output of the detectors was fed to a tuned 
amplifier, and the amplified output voltage was 
read on a Ballantine electronic voltmeter. A curve 
was plotted for each detector in which the ordinate 
was the ratio of the detector’s response to the 
Coblentz response and the abscissa wavelength in 
microns. This ratio was adjusted to a 2fi at 2.0 p and 
is proportional to the absorptivity. In some cases, 
a 2l i was not known, so the curves were adjusted 
to unity at 2.0 p. 

Minimum Detectable Signal 

A rectifier was built to operate an Esterline-Angus 
recording milliammeter from the output of the vari¬ 
ous amplifiers. With this device, it was possible to 
register both signal and noise on the same record. 
The Esterline-Angus recorder was also adapted to 
drive the prism table of the spectrometer in order 


to permit the use of the various detectors in record¬ 
ing spectra. 

The detectors were set up to view the black body 
and the outputs were recorded on the milliammeter. 
As the black body cooled to room temperature, the 
shutter was held open for about one-half minute at 
regular intervals. The signal was thus recorded 
above the noise. The last signal clearly discernible 
above the noise was taken to be the MDS. Curves 
for the various detectors are shown in Figures 6 to 
11, inclusive. 

85 THERMOPILES DEVELOPED IN 
SECTION 16.4 

8,5,1 The Harris Evaporated Thermopile 

In order to obtain a radiation thermopile with a 
quick response, the metals composing the junctions 
must be exceedingly thin. The previously re¬ 
ported 8 > 9 sputtered and . evaporated thermopiles 
have comparatively fast responses. A quite different 
design of evaporated thermopile 4 appears to be 
capable of somewhat greater responsivity, and the 
new folded thermopiles which were developed from 
these previous studies have a much faster response 
than conventional thermopiles. They compare favor¬ 
ably with other fast-responding infrared detectors. 

According to the original proposals, rapid re¬ 
sponse was to be attained by permitting a high rate 
of heat loss, still using thin deposits of metal on a 
thin backing support. The overall responsivity was 
to be maintained by crowding many junctions into 
a small area. This was to be accomplished by evap¬ 
orating metals about 1.5 X 10~ 4 centimeter thick 
onto folded strips of the insulating support material, 
with the hot junction at the fold and the cold junc¬ 
tion just behind or below the hot junction. Figure 13 
shows the structure of the thermopiles. The method 
of manufacture is fully described elsewhere. 3 

Construction Details 

Method of Evaporating the Metals. Previous 
studies 10 had shown that the resistivities of evap¬ 
orated bismuth and evaporated antimony were 10 
to 30 times as great as those of the massive metal. 
The new design would be impractical unless these 
resistivities could be appreciably reduced, as it is 
important that the thermopile resistance be low. 
Accordingly, the first task was to develop methods 





240 


FAR INFRARED DETECTING ELEMENTS 


for evaporating the metals so that the metal deposits 
obtained would have resistivities approaching those 
of the massive metals. 



Figure 13. Folded thermocouple stack (not drawn to 
scale). 

The following method of depositing bismuth and 
antimony upon cellulose nitrate by evaporation 
was adopted. A weighed quantity of the pure metal, 
previously cast in vacuum, was evaporated from 
electrically heated boats. A tantalum boat was used 
for bismuth and a molybdenum boat for antimony. 
The heater current was increased gradually during 
the process so that at the end the boat from which 
antimony had been deposited had reached a red 
heat and the other a yellow heat. The evaporation 
was carried on stepwise, first a layer of antimony 
and then a layer of bismuth, 25 minutes being re¬ 
quired for a complete evaporation process, which 
took place under a vacuum of 10' 4 mm Hg. 

The progress of the evaporation was followed 
with an “evaporation meter” situated in the vacuum 


chamber directly above the boat from which metal 
was being evaporated. This so-called evaporation 
meter is made from a glass strip with silver- 
chromium terminals at the ends. Leads are brought 
out of the bell jar from the terminals and connected 
to a “volt-ohmist.” As metal becomes deposited on 
the glass strip during the evaporation process the 
resistance decreases. The change in resistance may 
be calibrated to serve as a rough measure of the 
amount of metal deposited and of its resistivity 
at the end of the evaporation process. Such meters 
may be used again and again by simply removing 
the metal after evaporation by solution with con¬ 
centrated hydrochloric acid. 

Measurements of the Electrical Properties of 
Evaporated Bismuth and Antimony. Early experi¬ 
ments showed that the amount of metal deposited 
and its resistivity varied with the evaporation tech¬ 
nique as well as with the material upon which the 
metal was deposited. The results given below are 
for the metals deposited upon thin films of cellulose 
nitrate. A strip of the cellulose nitrate used had 
been weighed previously so that the resistivity of 
the metals could be calculated. The results shown 
in Table 1 were obtained using the technique de¬ 
scribed above. 

The bismuth deposits had a dull light-gray ap¬ 
pearance; the antimony deposits were a brilliant 
steel gray and showed a crystalline pattern to the 
unaided eye. 

Alloys of bismuth containing about 5 per cent of 
either cadmium or aluminum have a lower massive 
metal electric resistivity than pure bismuth. Al¬ 
though the thermal emf (versus copper) of either 
of these alloys is smaller, i.e., less negative than for 
pure bismuth, it might still be more profitable to 
use these alloys instead of pure bismuth for the 
negative half of the thermoelements. Only prelim¬ 
inary experiments were made in the evaporation of 


Table 1 


Metal 

Thicknesses 

(microns) 

Resistivity 

(ohm-cm) 

Average Limits 

Thermal emf vs. copper 
(pvolts/degrees C) 

Average Limits 

Bismuth 

0.73-1.58 

168 X lO" 6 

131-212 X10 -6 

—57.5 

—55.1 to —59.0 


1.64-2.14 

159 X 10- 6 


—56.1 



2.05-3.15 

130 X lO" 6 


—56.0 


Antimony 

0.48-1.66 

95 X lO" 6 

60-119 X lO" 6 

35.5 

33.1 to 38.6 





















































THERMOPILES DEVELOPED IN SECTION 16.4 


241 


alloys before it was discovered how to obtain thin 
deposits of bismuth as well as of antimony with the 
comparatively low resistivities given in Table 1. 
Accordingly, the study of evaporation of the alloys 
was not pursued further. 

The Backing Support for the Metals. The sup¬ 
porting material upon which the metal is deposited 
by evaporation should have the following prop¬ 
erties. 

1. It must be an insulator. 

2. The product of heat capacity, density, and 
thickness (thermal mass) should be smaller than 
that of the metals, if possible. 

3. It should be stiff at the thickness used. 

4. It must be capable of being bent sharply 
through 180 degrees without cracking. 

5. It must withstand the temperature and ther¬ 
mal shock of evaporation. 

6. It must have a low vapor pressure. 

Early investigations indicated that a rather ex¬ 
tensive study would be required to find the ideal 
material. Although cellulose nitrate is not the ideal 
material, it satisfies most of the requirements and 
was used until a more suitable material could be 
found. Cellulose nitrate films as thin as 1.75 X 10~ 4 
centimeter lend themselves easily to the various 
operations encountered here. 

The cellulose nitrate films were formed by the 
“knife method,” wherein the film solution was 
poured on a glass plate and spread accurately to the 
proper thickness with a spreader bar or “knife.” 
The films were then cut to 9x1.2 cm on the glass 
plate and floated off on water, ready to be mounted. 

Mounting or Folding the Cellidose Nitrate. In 


order to obtain a uniform fold of the cellulose 
nitrate film it was necessary to fold it over a “bar” 
which was removed after the fold had set. Various 
plastic materials were tried, which could be re¬ 
moved with solvents to which the cellulose nitrate 
is inert. However, better results were obtained using 
a nylon thread held taut by a spring, the thread 
being withdrawn mechanically after the fold had set. 

All subsequent operations were facilitated by 
mounting the lower ends of the cellulose nitrate 
folds on aluminum strips 9 inches long, 0.25 inch 
wide, and 0.0025 inch thick (see Figure 13). The 
aluminum strips were cut, straightened, anodized 
and then mounted in a holder which kept them taut. 
The distance (0.010 inch) from the top edge of the 
anodized strip to the fold of the cellulose nitrate 
film was controlled by the spacing of the (0.003- 
inch diameter) nylon thread. The adhesion of the 
cellulose nitrate to the anodized strip was effected 
by painting glyptal solution, three-fourths of the 
width, along the length and on both sides of the 
strip. If the painting is continued over the full 
width, breaks often occur at the film-aluminum 
edge. 

The holder (Figure 14), with the anodized strip 
and the nylon thread, was submerged in a vessel 
filled with water to _ the surface of which the 
cellulose nitrate film is transferred. The film was 
oriented so that its long axis coincided with that of 
the strip. The water was then drained from the 
vessel so that as the water surface receded the film 
followed and was left in the folded position adhering 
to the anodized strip. After the films on the strips 
had dried, the nylon thread was withdrawn and the 



Figure 14. Holder with anodized strip. 










242 


FAR INFRARED DETECTING ELEMENTS 



Figure 15. Evaporation frame for evaporated thermopile. 


strip with the attached film was ready to be trans¬ 
ferred to the evaporation frame. 

Frames for Holding Strips during Evaporation. 
The evaporation frames limit the deposition of each 
metal to one side of the folded cellulose nitrate strip 
and to an overlap of about 0.15 millimeter at the 
fold. Each frame consists of four main parts, illus¬ 
trated in Figures 15 and 16. (A) holds the strip 
taut yet extendible (by means of a spring); (B) 
serves as a supporting back for the strip; (C) is a 
gold mask 0.0015 inch thick which helps to hold that 
part of the film which extends beyond the edge of 
the anodized strip down against the supporting back 
(B), and which also confines the metal deposits to 


bands 2.5 millimeters wide with a bare space 0.7 
millimeter wide between the bands; (D) holds the 
gold mask in place. 

After one metal has been deposited, the frames 
are removed from the bell jar, parts B, C, and D 
are transferred to the opposite side of part A, and 
the second metal is evaporated. Evaporation is ac¬ 
complished on four strips simultaneously. From 
each strip 22 thermal elements 0.125 inch wide can 
be cut. 

Stacking and Mounting of Thermopiles. The 
folded strips were then cut along the bare spaces 
into individual thermoelements, using an instrument 
similar to a paper cutter. The resistance of each 



Figure 16. Evaporation frame for evaporated thermopile, assembled. 





























THERMOPILES DEVELOPED IN SECTION 16.4 


243 


element was measured with a special testing clamp 
the jaws of which grip the element 0.3 millimeter 
back from the fold. Those elements having resist¬ 
ances greater than 1.75 ohms were discarded. 



Figure 17. Device for stacking Harris thermoele¬ 
ments. 

The elements were then stacked in groups of 50 
in the “stacker” shown in Figure 17. Top and bot¬ 
tom electrodes are silver strips 0.1x0.25x0.005 


inch. The bottom silver electrode was put into the 
stacker first, then the elements were put into place 
individually, the edges of the folds coming up 
against a glass plate. As the stacking was continued 
it was often necessary to improve the alignment of 
some of the elements. After 50 elements had been 
stacked, and the top silver electrode inserted, the 
middle plate of the stacker was slid down and pres¬ 
sure applied through a screw until a minimum 
resistance, usually about 60 ohms, was obtained. 
The outer side and the back of the pile were painted 
with cellulose nitrate, thus cementing the elements 
into the finished pile. Figure 13 illustrates one of the 
elements and also shows how the elements are 
arranged in a completed thermopile. 

The cemented piles were then put into a “skele¬ 
ton” of the final housing and coated with gold 
“black.” 

The skeleton was inserted into the outer casing 
of the housing and the piles were ready for use. 
Figure 18 shows four stacks of piles in a skeleton 
designed for the Bureau of Ships; Figure 19 shows 
the skeleton in its case. 

Physical Characteristics of Harris Thermopiles 

Table 2 lists the physical data obtained on four 
Harris thermopiles designated A, B, C, and D. The 
two designated A and B were adjacent to each other 
in one housing and constituted the pair tested 
under Contract OEMsr-1168, while C and D were 
adjacent to each other in another housing. 



Figure 18. Four stacks of piles in skeleton. 


Figure 19. Skeleton in thermopile case. 























244 


FAR INFRARED DETECTING ELEMENTS 


Table 2. Physical dimensions and properties of Harris thermopiles* 



A 

B 

C 

D 

Length from “hot” to “cold” junction 

0.25 mm 

0.30 mm 

0.35 mm 

0.15 mm 

Width of each element 

2.5 mm 

2.5 mm 

2.5 mm 

2.5 mm 

Thickness, antimony 

0.6 X 10" 4 cm 

0.6 X 10- 4 cm 

0.6 X 10~ 4 cm 

0.6 X 10“ 4 cm 

Thickness, bismuth 

1.07 X 10- 4 cm 

1.07 X 10- 4 cm 

1.07 X 10" 4 cm 

1.07 X 10” 4 cm 

Thickness, cellulose nitrate 

1.7 X 10- 4 cm 

1.7 X 10" 4 cm 

3.2 X 10~ 4 cm 

3.2 X 10' 4 cm 

Number of junctions in pile 

50 

50 

50 

25 

Active receiving area of pile 

0.11 sq cm 

0.11 sq cm 

0.09 sq cm 

0.05 sq cm 

Thermopile resistance 

63 ohms 

84.5 ohms 

56 ohms 

32.5 ohms 


* All the receivers were covered with gold “black.” Thermal emf (measured) Bi-Sb = 92 volts per degree C. 


Performance Characteristics of the 
Harris Thermopile 

Static Characteristics. As discussed under “Static 
Method,” Section 8.3.8, the d-c responsivity of 
thermopiles A and B was measured under Contract 
OEMsr-1168. The d-c responsivity for these ther¬ 
mopiles was also found by those working under 
Contract OEMsr-1147 and included in the contrac¬ 
tor’s progress report, 3 along with the responsivity 
of two other thermopiles designated C and D. These 
values have been grouped in Table 3. 


Table 3. D-C Responsivities of Harris thermopiles. 



A 

B 

C 

Paral- 

Series Series lei 
D A-t-B C + D A + B 

OEMsr-1147 







Volts/watt 

0.40 

0.467 

0.52 

0.15 0.431 

0.386 

0.202 

Area (cm 2 ) 

0.11 

0.11 

0.09 

0.05 0.22 

0.14 

0.22 

OEMsr-1168 







Volts/watt 

0.345 0.375 






It may be seen that although the responsivities of 
A and B determined under OEMsr-1168 are some¬ 
what lower than those determined under OEMsr- 
1147, they are in substantial agreement. The respon¬ 
sivities of A, B, and C are very nearly identical, 
while the responsivity of D is much lower. In Table 
2, it may be observed that D has only half as many 
junctions as A, B, or C. The heat loss for D is greater 
than for A, B, or C because of the shorter distance 
between the hot and cold junctions. The respon¬ 
sivities obtained with the piles in series (Table 3, 
columns 5 and 6) serve as a check on the measure¬ 
ments and prove that the voltage generated is 
roughly proportional to the number of junctions. The 
ENI, defined under 8.3.3, was determined for ther¬ 


mopiles A and B by Contract OEMsr-1168 at the 
two frequencies 14.6 cycles and 27.6 cycles, using 
an amplifier sharply tuned to the two frequencies. 
The ENI values found are listed below in Table 4. 


Table 4. ENI values for the Harris thermopiles 
determined with 5 n to 14 n radiation. 



ENI(14.6 c) 

ENI (27.6 c) 

MENI (14.6 c) 

Thermopile 

(|XW) 

(nw) 

(nw) 

A 

0.2 

0.2 

0.009 

B 

0.25 

0.25 

0.01 


As was suggested in Section 8.3.3, when the gain 
and frequency pass band of the amplifier and the 
resistance of the thermopile are known, the output 
voltage to be expected from the thermal-agitation 
noise may be computed. At frequencies of 14.6 
cycles and 27.6 cycles, the observed noise was 27 db 
and 20 db, respectively, above the computed value. 
Moreover, the noise observed with the thermopile in 
the circuit was essentially no different from the 
noise observed with the thermopile shorted out. 
This would seem to indicate that most of the 
observed noise originates within the amplifier. If it 
were possible to eliminate the amplifier noise so that 
the only noise present were that of the thermopile, 
the noise level might be expected to be reduced by 
the corresponding factors. The ENI under these 
conditions, i.e., the MENI, would be as listed in 
Table 4, column 4. 

The frequency-response curves of thermopiles A 
and B, as determined by the method outlined under 
“Frequency Response,” Section 8.3.7, is shown in 
Figure 20. No time constant can be ascribed to the 
frequency response of these thermopiles as the 
curves are not of the proper shape. The measure- s 
ments on A and B reported by Contract OEMsr- 


















THERMOPILES NOT DEVELOPED IN SECTION 16.4 


245 


1147 and shown in Figure 21 are in good agreement 
with those shown in Figure 20. Although thermopile 
C has substantially the same d-c responsivity as A 
and B, its response to interrupted radiation, as 



FREQUENCY IN C 

Figure 20. Frequency-response curve for Harris ther¬ 
mopile. 


shown in Figure 21, is much less than that for A and 
B. This is because its cellulose nitrate backing is 
much heavier. Because of its greater heat-loss rate, 
D should be faster than C. That this is true is made 
evident from Figure 21 by the flatter response curve 
for this thermopile. 



Figure 21. Response of thermopiles to interrupted 
radiation. 


Measurements on the blackness of the receiving 
elements, made as described in Section 8.4 under 
“Frequency Response” and shown in Figure 22, 
reveal that the Harris thermopiles are uniformly 
black throughout the spectral range from 1 p to 14 p. 



Figure 22. Spectral response curve for Harris ther¬ 
mopile. 


86 THERMOPILES NOT DEVELOPED 

IN SECTION 16.4 

8 61 The Weyrich Vacuum 

Thermocouple 

The Weyrich thermocouple was not submitted to 
those working under Contract OEMsr-1168 by 
NDRC or by any other agency interested in infra¬ 
red detectors. This thermocouple has, however, 
been used extensively over a long period of years 
by infrared spectroscopists and is, therefore, impor¬ 
tant as a detector. The device is a thermopile with 
two junctions which may be used as the receiving 
elements for infrared radiation. Each of these junc* 
tions has a receiving area of 0.02 square centimeter 
coated with zinc black. The thermoelectric materials 
are two antimony-bismuth alloys. 

Two leads for each junction emerge through vac¬ 
uum seals, so that the junctions may be joined ex¬ 
ternally in series aiding or in series opposing or in 
parallel. The resistance of each junction was found 
to be about 10 ohms. 

To determine the d-c responsivity, the thermo¬ 
couple was connected to a Leeds and Northrup 
high-sensitivity galvanometer and illuminated by 
radiation of known flux density. The thermal emf 
produced caused a galvanometer deflection which 
was compared with that from 1 pv introduced into 
the circuit. From the area of the element (0.02 
square centimeter), the responsivity in volts per 
watt was evaluated, and found to be 0.29 volt per 
watt. 

The frequency-response curve given in Figure 23 
was obtained by means of a General Motors ampli¬ 
fier. A ballistic galvanometer method yielded a time 
constant, assuming one to exist, of 68 milliseconds. 

















246 


FAR INFRARED DETECTING ELEMENTS 


When the thermocouple was evacuated, the re- 
sponsivity increased by a factor of about 15, and 
the time constant, measured by the ballistic gal¬ 
vanometer method, increased by the same factor, 
that is, to about 1,000 milliseconds. 

An investigation of the blackness of this thermo¬ 
couple at various wavelengths of radiation revealed 
that the thermocouple is as uniformly black 
throughout the spectrum from 1 p to 13 p as the 
Coblentz thermopile with which it was compared. 

8 6 2 The Eppley Thermocouple 

An Eppley thermocouple of the type employed in 
the Farrand device developed for experimental use 
by the Bureau of Ordnance, Navy Department, was 
furnished, through BuOrd, to Contract OEMsr- 
1168 for testing. 

In a general way, this thermocouple is similar to 
the Weyrich type described, although it is prob¬ 
ably somewhat more ruggedly constructed and pre¬ 
sumably unevacuated. The element consists of two 
thermal junctions joined in series opposing, each 
junction being fitted with a blackened receiver, of 
area 0.01 square centimeter. The resistance was 
found to be about 5.8 ohms. The responsivity was 
determined in the same manner as for the Weyrich 
thermocouple, and was found to be 0.375 volt per 
watt. 



Figure 23. Frequency-response curves for Weyrich 
and Eppley thermocouples. 


The frequency response of the Eppley thermo¬ 
couple was investigated by the use of the General 
Motors amplifier and is shown in Figure 23. Sev¬ 
eral sets of measurements indicated that the fre¬ 


quency response was characterized by a time con¬ 
stant of 90 milliseconds. Spectral response curves 
revealed that this thermocouple is as uniformly 
black throughout the spectrum from 1 p to 13 p as 
the Coblentz thermopile used as a standard. 

863 The Schwarz Thermopile 

The design of the Schwarz thermopile, constructed 
by Adam Hilger, Ltd., London, England, and fur¬ 
nished for testing by Section 16.4 of NDRC, is sub¬ 
stantially different from that of other thermocouples 
or thermopiles examined. As indicated in Figure 24, 


SCHWARZ THERMOPILE 



Figure 24. Diagram of Schwarz thermopile. 

the elements are mounted in a cylindrical case 2.5 
centimeters in diameter and 5.0 centimeters long. 
The thermopile is also shown schematically in Fig¬ 
ure 25. Rods of two different metals, sharpened at 

METAL NO. 1 RECEIVER METAL NO-2 



Figure 25. Schematic diagram of Schwarz thermo¬ 
pile. 

the top end, are mounted in blocks. A thermocouple 
is made by welding a blackened receiver to the 
sharpened ends o~f these rods. The receiver has the 
dimensions 1x2 millimeters, that is, an area of 0.04 
square centimeter when used in pairs. A thermopile 
is then formed, as indicated in the diagram, of sev¬ 
eral such couples joined in series. The electrical 
connections are made through two binding posts 
fastened to the back end of the case containing the 
thermoelements. These thermopiles were mounted 
in the case behind a window made of fluorite and 
were operated in air at atmospheric pressure. The 
Schwarz thermopiles are quite delicate and must 
not be submitted to great mechanical shock. 

The resistance of the thermal junctions varies sub¬ 
stantially from element to element, since the re- 
































BOLOMETERS 


i 

- 


247 


sistance in any case depends essentially upon the 
manner in which the receiver is welded to the 
pointed rods. The resistances of the three Schwarz 
thermocouples tested under Contract OEMsr-1168 
(Nos. B4772/9, 11, and 12) were found to be 23.5 
ohms, 51.0 ohms, and 20.7 ohms respectively, while 
the resistances quoted by Hilger are 20 ohms, 35 
ohms, and 20 ohms. 

The d-c responsivity of the Schwarz thermopile 
was obtained exactly as in the preceding cases. The 
values are summarized in Table 5. 


Table 5. Responsivities in volts per watt. 



Upper pair 

Lower pair Hilger values 

No. B4772/9 

0.72 

0.75 

0.9 

No. B4772/11 

0.85 

1.00 

1.2 

No. B4772/12 

0.65 

0.73 

0.85 


The frequency response for the thermopile was 
investigated, and a curve showing the observed re¬ 
sponse, converted into decibels, plotted against the 
logarithm of the frequency in cycles is shown in 
Figure 26 for the upper pair of unit No. 11. The 
zero-frequency response in decibels was also meas¬ 
ured and is shown by the arrow in Figure 26. The 



Figure 26. Frequency-response curve for Schwarz 
thermopile. 


curve shown is quite typical for all of the Schwarz 
thermopiles and it shows that the frequency re¬ 
sponse is not such that a time constant can be as¬ 
cribed to it. 

The ENI was determined at the two frequencies, 
14.6 cycles and 27.6 cycles, using a tuned amplifier 
and a square-wave heat input. The first two col¬ 
umns of Table 6 give the ENI values for all the 


pairs of the three thermopiles. These ENI values 
are for Nernst glower radiation, which is principally 
2 p. Radiation of this wavelength was used because 
the fluorite window of the thermopile is opaque be¬ 
yond 10 p. The last column of Table 6 gives the 
MENI, which is the optimum value for the ENI. 

A relative spectral response curve for thermopile 
No. 12 with the ratio of the Schwarz response to the 
Coblentz response adjusted to unity at 2.0 p is 
shown in Figure 27. It is readily seen that the rela- 



Figure 27. Spectral response curve for Schwarz ther¬ 
mopile. 


tive response falls off materially beyond 6 p and 
becomes quite small at longer wavelengths. This 
decrease from 6 p on can be ascribed to the fluorite 
window used with the thermopile. From 1.0 p to 5.0 
p the relative response is very nearly unity. 


Table 6. ENI observed in the 2-p spectral region. 




ENI 

(pw) 

14.6 cycles 

ENI 

(pw) 

27.6 cycles 

MENI 

(pw) 

14.6 cycles 

No. B4772/9 

Upper 

0.035 

0.05 

0.004 


Lower 

0.04 

0.05 

0.005 

No. B4772/11 

Upper 

0.075 

0.09 

0.009 


Lower 

0.075 

0.08 

0.009 

No. B4772/12 

Upper 

0.05 

0.037 

0.006 


Lower 

0.06 

0.05 

0.008 


87 BOLOMETERS 

Metal Strip Bolometers 

A metal' strip bolometer is a device which, by 
means of a large temperature coefficient of resistance, 
is capable of detecting very small amounts of radi¬ 
ant energy. It is a low-resistance element and is made 
of very thin material, usually by an evaporation 
process. As developed under Contract OEMsr-60 11 
with Harvard University, a metal strip (Strong) 













248 


FAR INFRARED DETECTING ELEMENTS 


bolometer consists of one or more strips mounted 
within a metal case behind a window capable of 
transmitting infrared radiation. The bolometers 
usually operate in hydrogen at reduced pressures. 

When used to detect heat radiation, two bolom¬ 
eters are generally connected in series to the pri¬ 
mary of a transformer, the secondary of which is 
capable of matching the impedance of the first tube 
of the amplifier (Figure 28). The center tap of the 




POLAROID INPUT 

Figure 2S. Input circuit for Strong and Polaroid 

bolometers. 

transformer primary is connected through a bat¬ 
tery to the junction of the two strips to furnish the 
appropriate voltage across the bolometer. When in¬ 
terrupted radiation falls on one of the bolometers, 
an alternating emf is generated and impressed on 
the grid of the first tube by means of the trans¬ 
former. 

Production of Bolometer Strips 

Nickel strips have been used effectively for bo¬ 
lometers. The method of preparing them (Contract 
OEMsr-60) involves a composite coat: silver, alu¬ 
minum, and nickel are deposited, successively, by the 
evaporation process, on the clean freshly broken 
edge of a glass plate. Later, the composite layer is 
separated from the glass, and the other metals are 
dissolved from the nickel. Strips formed on such 
edges are characterized by sharp unbroken borders. 

The various steps in making a nickel bolometer 
strip are outlined in the paragraphs which follow. 

The more or less straight edge of a broken glass 
plate, of 0.5 to 1 millimeter thickness, is coated with 
a thin transparent film of silver to provide a base 
layer which later can be stripped off the glass. 


Next, a deposit of aluminum, of 0.5 to 1 p thick¬ 
ness, is formed over the silver. The purpose gf this 
aluminum deposit is to afford a means by which the 
nickel can be freed of silver, and also to give the 
strip strength during the operation of separating it 
from the glass by peeling. 

Finally, an evaporated coat of nickel, of about 
0.25 p thickness, is deposited over the aluminum 
layer. 

The composite three-film coat is peeled off the 
glass edge with tweezers. The operation is facili¬ 
tated by applying a drop of water which contains 
a wetting agent. The composite strip is then dipped 
in a beaker containing caustic solution. The alumi¬ 
num interfilm dissolves in this solution and carries 
the silver away at the same time. The remaining 
nickel strip is rinsed several times with water until 
all the caustic is removed. The nickel strip is finally 
removed from the rinse water on a strip of paraffin 
paper 0.5 inch wide, on which it is allowed to dry. 

Bolometer Construction 

A bolometer is made by attaching one or more 
nickel strips to appropriate terminals by means of 
soft solder, as follows: The terminals are first sep¬ 
arately tinned with a small soldering copper, then 
they are wetted with soldering flux by means of a 
fine ink pen; the strip is laid across the terminals 
and adjusted in place—the flux acts as a lubricant 
and facilitates the adjustment of the strip: finally, 
each terminal is slowly heated with the small sol¬ 
dering copper until the flux dries and the end of the 
strip becomes “wetted” with the solder on the 
terminal. 

Mounted strips are coated with aluminum black 
by letting evaporated aluminum black (aluminum 
evaporated from a tungsten coil under a hydrogen 
pressure of 3 millimeters of mercury) settle onto 
them. 

T Ymdoics. Rolled sheets of AgCl, of -inch 
thickness, have been adopted as window material 
for bolometers. They are attached with Sealstix ce¬ 
ment or glvptal lacquer and are protected from 
solarization by means of a coat of gilsonite (after 
Pfund) which is dissolved in benzene and applied 
to the window with a fine brush. A thin uniform 
coat of gilsonite, which does not attenuate the vis¬ 
ual transmission of the AgCl by more than 50 per 
cent, effectively prevents its decomposition by the 
action of light. 




























BOLOMETERS 


l 


249 


It is necessary to avoid a direct contact between 
AgCl sheets and brass, since on contact a reaction 
between these materials sets in which results in the 
complete disintegration of the AgCl. To avoid this 
reaction, the window frame of a brass bolometer can 
be formed from a layer of fine silver, which may 
be either soldered in place or deposited by electro¬ 
plating. AgCl is stable in contact with fine silver. 

Bolometers made from 0.25-p nickel strips and 
intended for operation at about 40 cycles are evac¬ 
uated and then filled with hydrogen at a pressure 
of about 1 millimeter of mercury. At this pressure 
the conductive cooling by this gas is still high, while 
the convective cooling is attenuated some 760 times 
from its value for atmospheric pressure. Since varia¬ 
tions of convective cooling, at atmospheric pressure, 
give rise to strong spurious microphonic signals, 
whereas the conductive cooling at low pressures 
does not, this choice of pressure is advantageous. 
The use of a separate charcoal chamber provides a 
means for absorbing other gases which are absorbed 
more strongly than hydrogen by the charcoal. This 
separate chamber can be made of brass or glass, and 
attached to the bolometer, remotely, by means of 
%-inch outside diameter copper tubing. During 
evacuation of the bolometer, the trap is advan¬ 
tageously baked out at a temperature of about 
300 C. 

Application of the Nickel Carbonyl Process to 
Bolometer Construction 

The manufacture of metal or nonmetal bolome¬ 
ters in quantity presents problems because of their 
delicacy. When this was realized, a method of con¬ 
struction was devised and tested which promises to 
avoid some of these problems. By this method the 
bolometer strips (of any material not attacked by 
CO) are formed on a nickel base which is later 
removed by reaction with CO to form Ni(CO) 4 . 
The list of metal strip materials to which the method 
may be applied excludes nickel (and, to a certain 
extent, iron) but gold or rhodium are suitable. Non- 
metal strips or other thin films, for example, thin 
quartz films, have been made by this process. 

The metal and nonmetal bolometer strips which 
have been made by this process under Contract 
OEMsr-60 include gold, nickel oxide, Ag 2 S, and 
quartz. They have not been made at ordinary pres¬ 
sure but rather at 200 atmospheres pressure of pure 
CO at 200 C. The process was carried out in heavy- 


walled glass containers. The thin unsupported 
quartz films were made by first evaporating quartz 
on a heated nickel backing to obtain the quartz 
film in an annealed condition. 

The advantage of this carbonyl method of mak¬ 
ing strips and films lies in the fact that the com¬ 
posite starting strips are sturdy during all the opera¬ 
tions of construction; and further, the nickel back¬ 
ing is finally removed without even subjecting the 
bolometer material to the forces of surface tension 
or to the tearing action of bubbles formed, as when 
a surrounding metal is removed by solution in acid 
or alkali reagent. 

Both Ni(CO) 4 and CO are very poisonous and 
suitable precautions must be taken to prevent 
breathing these gases. 

Tests of the Strong Bolometer 

The one metal-strip bolometer unit constructed 
and furnished for testing by Harvard University 
under Contract OEMsr-60, consisted of six blacked 
nickel strips connected as shown in Figure 28. Each 
strip is about 1.2x4.8 millimeters and thus about 
0.057 square centimeter in area. Because of the re¬ 
sistance of the transformer, wires, and current¬ 
measuring equipment, the working voltage across 
three strips of the bolometer was about 1 volt. The 
strips were mounted and encased as previously de¬ 
scribed. 

Static Characteristics of the Strong Bolometer. 
By a voltmeter-ammeter method, the resistance of 
the bolometer strip was measured while its tempera¬ 
ture was varied, the temperature being determined 
with a calibrated thermocouple. From the slope of 
a graph of R b versus T, the temperature coefficient 
of resistance a was found to be 0.0045 per degree 
centigrade. 

The variation of the bolometer resistance as a 
function of the d-c power input was measured by 
means of a Wheatstone bridge powered by a storage 
battery with a variable resistance in series. The 
power to the bolometer strip was varied by chang¬ 
ing the current supplied to the bridge. As the cur¬ 
rent in the bolometer was also measured, the power 
expended in the strip could be computed for each 
resistance measurement. Figure 12 shows the graphs 
of R b versus P and R b versus I b . The quantity 
dR b /dP for any operating current was obtained 
from the R b versus P curve of Figure 12. In the tests 
reported here the bolometer current was 0.24 am- 






250 


FAR INFRARED DETECTING ELEMENTS 


pere, and dRJdP for this current was about 6.6 
ohms per watt. 

As not all of the heat power which fell on the 
bolometer was effective in causing resistance change 
and because radiation of different wavelengths was 
not necessarily equally effective, it was not suffi¬ 
cient to measure only dR b /dP. The resistance 
change per watt of heat power dR b /dH or the ab¬ 
sorptivity ax had also to be determined. By direct 
measurement (see Section 8.3.8) dR b /dH for 2 p 
radiation was found to be 2.54 ohms per watt, and a* 
for 2 p radiation had the value 0.38. 



Figure 29. Absorptivity curve for Strong bolometer. 

The spectral response of the bolometer from 1 p 
to 10 p, measured with a Hilger rock salt prism 
spectrometer, was compared with the response of 
the Coblentz thermopile over the same spectral 
range. The ratio of the bolometer response to Cob¬ 
lentz response, arbitrarily fixed at the absorptivity 
value 0.38 at 2 p, was plotted versus wavelength 
and is shown in Figure 29. The absorptivity in¬ 
creased by about a factor of 2 in going from 2 p to 
6 p. This may be explained on the assumption that 
the transmission of the silver chloride window was 
appreciably better at 6 p than at 2 p, as the silver 
chloride had been exposed to light for some time. 

It is not known whether this measured increase 
in absorptivity can be considered reliable, because 
it was also found that the voltage output per watt of 
heat input in the 7-p to 14-p wavelength range was 
not so much greater than the same relation in the 


2-p region as the absorptivity curve would suggest. 
The receiver was quite uniformly black throughout 
the region investigated. During the course of the 
experiments, the original silver chloride window was 
replaced with one of polished rock salt. An increase 
of voltage output by a factor of 2.5 was noted in the 
2-p region. Rock salt has, in general, a more uniform 
and higher transmission than silver chloride 
throughout the infrared range. 

Dynamic Characteristics of the Strong Bolometer. 
The frequency response of the Strong bolometer 
was observed from 1 cycle to 250 cycles by a method 
similar to the one described under “Frequency 
Response,” Section 8.3.7, and under Section 8.4. A 
curve was made for each one of the elements and, 
as they proved to be nearly identical, an average 
response curve was drawn and is shown in Figure 30. 
The response for 1 cycle to 40 cycles was measured 
separately from the 10-cycle to 250-cycle range, the 
latter being the more reliable. The two curves do 
not have quite the same slope in the range where 
they overlap, but the indicated 3-db drop from zero- 
frequency to 10 cycles is not an unreasonable one. 
As the asymptote approached by the curve at high 
frequencies, was somewhat steeper than that to be 
expected if the bolometer had a time constant, none 
has been computed. The behavior may be read 
directly from the curve. 

to 20 40 60 80 too 200 400 600 



Figure 30. Frequency-response curves for Strong bo¬ 
lometer. 


The ENI was measured for a variety of fre¬ 
quencies by a method described in “Heat Sources,” 
Section 8.4. An amplifier which could be made to 
tune sharply at frequencies 14.6, 27.6, 47.0 and 80 








BOLOMETERS 


/ 


251 


cycles was used for these measurements. The values 
obtained are given in Table 7. In column 4 may be 
seen the effect of the replacement of the silver 
chloride window with rock salt. In addition to the 
ENI, values of the minimum detectable signal were 
obtained as outlined under “Minimum Detectable 
Signal,” Section 8.4. A record of signal and noise 
on the same chart taken at 27.6 cycles is shown in 
Figure 6, indicating a signal just above the noise. 

Table 7. The ENI values for the Strong bolometer. 


The ratios of the predicted output to the measured 
output for any given frequency are found in col¬ 
umn 8. It may be seen that in the case of poorest 
agreement the ratio is 1.7 and for the best the ratio 
is 1.13. This is considered to be good agreement. 

Later gain measurements indicate that the tabu¬ 
lated values are high and that better agreement 
between the expected and measured output would 
probably have been obtained with the later amplifier 
gain measurements if the bolometer measurements 
had been repeated. 


ENI 
(pw) 
electric 
black body 
Frequency (AgCl 

(cycles) window) 


ENI 

(pw) 

NBS standard lamp 

AgCl NaCl 
window window 


MENI 

(pw) 

AgCl 

window 


14.6 

0.03 

0.04 

0.02 

0.005 

27.6 

0.06 

0.08 


0.009 

47.0 


0.10 

0.03 


80.0 

0.14 

0.13 

0.04 

0.035 


Calculations of the thermal agitation noise in¬ 
herent in this bolometer were made as outlined in 
Section 8.3.2 and were found to be 12 to 18 db less 
than the observed noise output, depending upon the 
frequency. The output noise was 0.3 to 0.35 volt for 
all frequencies used with the tuned amplifier. If the 
noises arising in the amplification system could be 
reduced so that the thermal agitation noise of the 
bolometer was the only important contribution to 
the total noise output, then there would be a cor¬ 
responding improvement in the ENI. This MENI, 
computed from the experimental ENI values ob¬ 
tained for 5-p to 14-p radiation, is given in Table 7. 

From the static characteristics and frequency re¬ 
sponse of the bolometer and the gain of the ampli¬ 
fier, the output voltages at the frequencies of 14.6, 
27.6, 47.0, and 80 cycles have been predicted and 
are found in column 6 of Table 8. In column 7 are 
shown the values of the output actually measured. 


Thermistor or High-Impedance 
Bolometers 

Thermistor material is a semiconducting sub¬ 
stance which has a large negative temperature co¬ 
efficient. A thermistor is, therefore, a resistor, the 
resistance of which changes rapidly with tempera¬ 
ture. A bolometer constructed of this material is a 
device which may be used to detect or measure 
small quantities of radiant heat energy which cause 
a temperature rise in the material. As finally de¬ 
veloped under Contract OEMsr-636 with BTL, a 
typical thermistor bolometer consists of one or more 
flakes of thermistor material cemented on a back¬ 
ing which in turn is in good thermal contact with a 
metal case having a window which permits infrared 
radiation to pass through it. The sensitive flakes 
are from one to several millimeters long, a few 
tenths of a millimeter wide, and about 10 p thick. 

When used as a bolometer, one or more of these 
flakes are connected electrically in one or more 
arms of a single Wheatstone bridge circuit. Elec¬ 
trical current is passed through the flakes and two 
contacts of the bridge are connected to the input 
of a high-gain amplifier. This multiplies the elec¬ 
trical voltage developed in the bridge and delivers 
it to various forms of indicating devices, such as 
meters, recorders, cathode-ray tubes, or headphone. 


Table 8. Expected outputs and measured outputs for Strong bolometer. 


F requency 
(cycles) 

G r 

ilRb/dH 
(ohms/watt) 

h 

(amp) 

0 

Expected 
dE out/ dH 
(volts/pw) 

Measured 
dEout/dH 
(volts/p w) 

Ratio 

14.6 

1.55 X 10 8 

2.54 

0.24 

0.684 

29.2 * 

17.2 

1.7 

27.6 

0.80 X 10 8 

2.54 

0.24 

0.537 

11.8 

8.2 

1.44 

47.0 

0.80 X 10 8 

2.54 

0.24 

0.335 

7.35 

6.5 

1.13 

80.0 

1.40 X 10 8 

2.54 

0.24 

0.182 

6.91 

4.3 

1.61 











252 


FAR INFRARED DETECTING ELEMENTS 


When no radiant energy falls on the bolometer, the 
bridge is usually balanced and the indicating device 
is quiet. However, when radiant energy falls on one 
of the bolometer flakes, an increase in temperature 
is produced and the bolometer resistance is reduced. 
The bridge is thereby unbalanced and delivers a 
small voltage to the amplifier which in turn oper¬ 
ates the indicating mechanism. 

Such a bolometer and amplifier combination is 
capable of detecting about 0.1 pw in several milli¬ 
seconds. The temperature of the flake is increased 
by about a 10" 6 C, and the amplifier receives about 
1 pv. The resistance of a flake is about a megohm 
and about 100 volts are required to operate it. 
Since these values depend on the area of the bolom¬ 
eter and on its operating conditions, they are meant 
to convey orders of magnitude only. 

How Thermistor Bolometers Are Made 

It is desirable in practice to form the thermistor 
element first and then attach it to the backing 
proper for the particular application for which it is 
to be used. The following discussion of the forma¬ 
tion and assembly of the bolometer has been con¬ 
densed from the report 12 on this subject issued 
under Contract OEMsr-636. 

In order to make the resistance of the bolometer 
element conform to a desired value for any par¬ 
ticular size and shape, a proper choice of material 
is necessary. For example, one or more of the oxides 
of manganese, nickel, cobalt, or copper may be used. 
The majority of units requested to date have called 
for a resistance in megohms, lengths from 1 to 6 
millimeters, and a thickness of about 10 p. A com¬ 
bination of the oxides of manganese, nickel, and 
cobalt has been found suitable for meeting these 
requirements. In a few cases the oxides of manga¬ 
nese and nickel have been used. 

In order to provide a thin and sufficiently homo¬ 
geneous film of resistance material, attention must 
be paid to the particle size of the material used. It 
has been found necessary to employ a powder hav¬ 
ing a maximum particle size of about 0.01 the 
desired thickness of the completed flake. Mixed 
oxides of this particular size, a temporary binder, 
and a volatile solvent were used. The temporary 
binder most commonly used*was polyvinyl butyral, 
the solvent a mixture of ethyl alcohol and amyl 
acetate. These materials were placed in a ball mill 
and ground to a homogeneous liquid mix. A few 


drops of this material were then placed on a flat 
glass surface and spread to a uniform thickness by 
a properly spaced straightedge. The solvent was 
then allowed to evaporate in a dust-free atmos¬ 
phere. The dried film was removed from the plate 
by dampening with water and stripping from the 
surface of the glass. After being cut to the desired 
size, each flake was placed on a thin plate of plati¬ 
num which had been coated with a thin layer of 
aluminum oxide, the purpose of which was to pre¬ 
vent the flakes from sticking to the plate during 
the subsequent firing operation. 

The temperature of the furnace was gradually 
raised from room temperature through 400 C and 
finally to some point between 1100 and 1400 C to 
complete the sintering, depending on the material 
from which the flakes were made. When cool the 
flakes were removed by a suction device and stored 
in a suitable container. At this point they were 
ready for attachment of terminals and subsequent 
mounting. Up to this point in the process, practi¬ 
cally all the operations were done in multiple; there¬ 
after the operations were performed individually 
with some emphasis given to manipulative dexterity. 

To each end of the flake an electrode was then 
attached. This was done by applying a paint 
(platinum black No. 505 made by Hanovia Chemi¬ 
cal and Manufacturing Company) to the contact 
area, drying out the oil of rosemary solvent, and 
firing to some temperature between 800 and 860 C. 
The painting operation was done under a micro¬ 
scope with a small stylus. 

Care was taken in the painting operation to wet 
the desired area thoroughly and so produce a well- 
defined line between the contact and the active area 
of the flake. After being fired, the contact area had 
a continuous metallic coating which was in very 
intimate contact with the thermistor substrate. This 
was essential to insure contact with a definite 
active area over the range of current and tempera¬ 
ture for which the flake was to be used; otherwise, 
various types of electric noise render the flake 
useless. 

Following the application of the contact, one end 
of a 0.001-inch diameter platinum wire lead was 
fastened to it by means of a platinum-silver alloy 
paste. A suitable material for this purpose is a 
platinum alloy paste, No. 18, also made by Hanovia. 
This material has a somewhat higher flux content 
than the No. 505 and is also less rich in platinum. 






BOLOMETERS 


l 


253 


The paste securing the lead was fired as given above 
for the contact. 

When the terminals were attached to the flake, 
the active area and resistance were completely fixed. 
If the flakes were to be used in bolometer units 
requiring that the active areas and/or resistance of 
two or more flakes be matched, the desired resist¬ 
ance and dimensions were determined and filed with 
each individual flake so that desired matching could 
be done. Up to the present time, all requirements 
of width of active area had been met without selec¬ 
tion. The length requirements are met by a very 
large percentage of flakes as now produced. In 
cases where the resistances of two (and in one 
application four) of the flakes used to form a bolom¬ 
eter unit were to be closely matched so they could 
be incorporated in bridge circuits, a selection was 
made from a comparatively large stock. 

After such matching as was required by the 
design of a particular bolometer in which the flakes 
were to be used, they were cemented to a backing 
structure. The cement most frequently used for this 
operation was bakelite resin BR-0014. This is a 
thermosetting cement whose principal ingredient 
is the uncured resin. Since its thermal conductivity 
was more than tenfold lower than that of the therm¬ 
istor flake material or of most backing materials 
of practical interest, the thickness of this cement 
layer was of considerable importance. It was gen¬ 
erally -desirable to have it as thin as practicable. 6 
If the layer was too thin, however, poor mechanical 
contact between the flake and the backing resulted. 
The flake was applied to the backing as follows. 

A camel’s hair pencil brush was used to apply a 
properly thinned coat of resin to the backing mate¬ 
rial. The back of the flake was then wetted with a 
drop of similar material and, while the backing was 
still wet, the flake was applied to it and manipulated 
back and forth to give closer contact. The alcohol 
solvent was then dried out of the cement by baking 
the assembly at 80 C for 16 hours. During this 
period partial curing of the bakelite resin also took 
place. The complete curing was effected by a two- 
hour baking at 125 C. 

In most applications the backing consisted of a 
block of solid material of which the mass was large 

c In one application of importance, a layer of paper 
0.0004 inch thick was first attached to the quartz backing 
with the BR-0014 cement. The purpose of the paper was to 
serve as a spacer to make the effective cement layer thicker. 


compared with that of the flake. The backing mate¬ 
rial was selected on the basis of the time constant 
desired for a particular application. Where low time 
constants are desired the backing must have high 
thermal diffusivity. Backing materials used in vari¬ 
ous units to date are quartz, glass, anodized alumi¬ 
num, silver with a thin electrically insulating layer 
of mica interposed between the flake and the metal 
block, etc. Unbacked units have also been made in 
which the cooling agent has been air or helium, 
occasionally at less than atmospheric pressure. In 
unbacked units a supporting structure has been used 
in lieu of the backing. One such structure has been 
a set of 0.020-inch diameter wires sealed into a 
glass bead to serve as “posts” to which the terminal 
wires of the flake can be attached. Another has been 
made by undercutting one of the above backing 
structures just beneath the active area of the flake. 
The backed flake assembly was then fastened into 
the base of a housing or capsule. This was done by 
fastening the backing surface and housing together 
with cement (No. 624N, B. B. Chemical Company). 
The principal adhesive component is synthetic 
rubber. 

This base was in each case designed to fit the 
equipment into which it was to be subsequently 
mounted. All housing bases, however, consisted 
essentially of a metal shell into which the necessary 
number of lead wires were hermetically sealed to 
accommodate the flake assembly. A cap covered 
over the base to form the complete capsule. This 
cap consisted of a silver element into which a silver 
chloride window was welded after treatment with 
gilsonite or silver sulfide to make it opaque to 
visible and ultraviolet light. 

The above description is generally applicable to 
bolometer units. One unit, the Penrod type, has been 
chosen to illustrate a particular assembly. Figure 31 
shows the component parts (roughly 1% times actual 
size) and indicates the steps in its assembly. The 
large black strip to the left is the thermistor sheet 
before firing; the two adjacent pieces have been 
cut from it; the next two strips have been fired. The 
next two strips have platinum contacts and leads; 
the strips are next shown on the backing block 
prior to insertion into the mount. 

This mount is shown in four stages of assembly 
on the upper layout. The three pins to the left are 
glass-covered cunife wires which are inserted and 
cemented into the three holes visible at the end of 








254 


FAR INFRARED DETECTING ELEMENTS 


the first view, as shown in the second view. The 
third view of the mount shows the thermistor flakes 
with their backing block in place. The platinum 
leads are soldered to the cunife wire ends. Below 
the third view, starting at the bottom of the window 
sections, is the silver chloride disk, the silver win¬ 
dow frame, then the two welded together. The 
window area is shown coated with gilsonite. The 
final assembly at top right is a completed unit with 
silver window frame soldered to the main body. 


thermistor flakes were mounted parallel with each 
other and close together on the same backing. 
Table 9 lists the information furnished by BTL 
on the bolometers sent for testing. The necessity 
of completely terminating Contract OEMsr-1168 
by October 31, 1945, made it impossible to test all 
of the BTL bolometers completely. The informa¬ 
tion obtained on bolometers, for which measure¬ 
ments were completed, is contained in Table 14. 
The interpretation of the sensitivity symbols will 



Figure 31. Component parts of Penrod type bolometer. 


Tests on BTL Thermistor Bolometers under 
Contract OEMsr- 1168 2a 

Through Section 16.4 of NDRC a number of BTL 
thermistor bolometers were sent to Ohio State 
University for testing. Two kinds of thermistor 
material were used. The bolometers made from one 
kind had resistance from 20 to 30 megohms at room 
temperature; from the other, 3 and 4 megohms. 
They were single and double-element types intended 
for push-pull or balanced operation. Within each 
class were unbacked designs and others backed by 
glass or quartz. In the double element design the 


be considered later under “Sensitivity.” In Table 9 
the bolometers are listed according to material, 
backing, and number of elements. 

An amplifier used in these tests was built by the 
Western Electric Company, BTL Contract OEMsr- 
1098, 17 especially for use with thermistor bolom¬ 
eters. It was a high-gain design using twin-T feed¬ 
back networks to obtain a very sharp pass band 
(noise pass-band width of 2 cycles) centered at 
15 cycles. The amplifier output was available both 
as a 15-cycle voltage and as a rectified current suffi¬ 
cient to operate a 5-milliampere Esterline-Angus 
recording meter. Though the gain of the amplifier 










BOLOMETERS 


255 


Table 9. Description of BTL bolometers. 

Bolometer 

Width 

(mm) 

Separa- 

Length tion 

(mm) (mm) 

Area 
(sq cm) 

/ 

t (mse6) 

100 V 

(v/w/sq cm) 

(v/v) 


No. 1 

Material, air-backed, one-element 

type with 

NaCl window 



XB-241 

0.207 

3.10 

0.00642 


1.55 

242.0 

XB-242 

0.206 

2.98 

0.00614 


1.29 

210.0 


No. 2 

Material, air-backed, one-element 

type with 

NaCl window 



XB-108 

0.199 

2.91 

0.00579 


1.36 

234.0 

XB-239 

0.195 

3.00 

0.00585 


1.32 

226.0 

XB-240 

0.203 

3.00 

0.00609 


1.32 

216.0 


No. 2 Material, glass-backed, one-element type with NaCl window 



S-19 

0.202 

2.981 

0.00602 

5.9 

0.938 

156.0 

XB-237 

0.202 

3.00 

0.00606 

7.6 

0.675 

111.0 

XB-238 

0.194 

3.03 

0.00588 

7.6 

0.738 

126.0 


S-20 

XB-235 

XB-236 


No. 2 Material, quartz-backed, one-element type with NaCl window 

0 207 3.017 .... 0.00625 3.1 0.415 

0 200 2.95 .... 0.00590 3.0 0.329 

0 201 2 96 .... 0.00595 3.0 0.366 


66.5 
66.0 

61.5 


No. 2 Material, glass-backed, two-element type with AgCl window (the AgCl has sulfide on the top side) 


PND-108 left 

0.202 

2.951] 

0.401 

0.00596 

10.1 

0.637 

PND-108 right 

0.203 

2.9641 

0.00602 

8.6 

0.622 

PND-118 left 

0.195 

3.045] 

| 0.403 

0.00594 

8.6 

0.646 

PND-118 right 

0.197 

3.020! 

0.00595 

8.6 

0.676 


107.0 

103.0 

109.0 

113.0 


No. 2 Material, quartz-backed, two-element type with AgCl window (the AgCl has sulfide on the top side) 


PND-104 left 

0.195 

2.963 

PND-104 right 

0.194 

3.021 

PND-125 left 

0.193 

3.085; 

PND-125 right 

0.195 

3.060 


0.00578 

5.2 

0.558 

96.6 

0.00586 

5.2 

0.551 

94.1 

0.00597 

3.7 

0.382 

64.1 

0.00597 

5.7 

0.687 

115.0 


was adjustable in 2-db steps, a gain of 118 db from 
the grid of the first tube to the output measuring 
instruments was usually employed. 

The bolometer and leads to the amplifier were 
mounted in a 0.25-inch diameter copper tube filled 



with paraffin with the head protruding for irradia¬ 
tion. This arrangement served not only to shield the 
input circuit but also to prevent microphonic pickup. 
The input circuit is shown in Figure 32. The heat 


power to be detected was chopped at 15 cycles by a 
sector wheel and allowed to fall on the bolometer 
R. The 15-cycle changes in the bolometer resistance 
produced corresponding voltage changes which were 
amplified by the tuned amplifier. 

Static Characteristics of Thermistor Bolometers. 
The resistances of the various elements were meas¬ 
ured with a Wheatstone bridge as the temperature 
was varied over the range 0 to 60 C. It was found 
that the resistance could be quite accurately repre¬ 
sented by an equation of the type 13 R = A exp 
(P/T) where /? is a constant (Figure 33). This is 
typical of semiconductors. The resistance of the 
bolometer elements was also measured as the applied 
voltages and current were varied. From these meas¬ 
urements the curves shown in Figure 34 for XB-108, 
S-19, and S-20 were drawn. These curves, which are 
substantially identical with those furnished by BTL 
under Contract OEMsr-636, are representative, 





















256 


FAR INFRARED DETECTING ELEMENTS 


hence curves for the others are not shown. The 
curves indicate the interdependence of voltage, cur¬ 
rent, resistance, and power when the bolometer in 
its housing is at room temperature. From the data 



1.5 I------1 

20 25 30 35 40 45 50 


TEMPERATURE IN DEGREES C 

Figure 33. Curves showing static characteristics of 

BTL bolometers. 

furnished by such curves as are shown in Figures 
33 and 34, it was observed that dT/dP was a con¬ 
stant for each bolometer. The values of and 
dT/dP for the bolometers are found in Table 10. 

In Figure 35 is shown the exponential heating and 
cooling curve for an unbacked bolometer with sensi¬ 
tivity plotted as ordinate and time plotted in sec¬ 
onds as abscissa. 

The PND bolometers consist of two bolometer 
flakes mounted on the same piece of backing mate¬ 
rial. The temperature of each one of the flakes will 
tend to increase with an increase in the temperature 
of the other. It is easily seen that for identical (or 
similar) flakes dT 1 /dP 2 = dT 2 /dP 1} where the sub¬ 
scripts distinguish the two flakes, regardless of the 
flake material and size and regardless of the nature 
of the backing material. This equality was experi¬ 
mentally verified. 

By changing the amount of electrical power input 
to one flake while measuring the resistance of the 
other flake it was possible to measure dR 1 /dP 2 . 
The known R — T relation for the flakes then 


permitted a determination of dTJdP 2 . The stati¬ 
cally determined values of dTx/dP 2 were: for 
PND-108, 184 C per watt; for PND-118, 167 C 
per watt; and for PND-125, 83 C per watt. 

It is to be expected that the ability of one flake 
to affect the other would depend upon the frequency 
at which the flakes are heated. The frequency re¬ 
sponses show that this coupling between the flakes 
is negligible at the operating frequency of 15 cycles. 
The coupling is important, however, in establishing 
the d-c operating temperatures. 


Table 10. Values of (3 and ( dT/dP) for BTL 
bolometers. 


Bolometer 

P (°K) 

dT/dP 

(°C/watt) 

XB-108 

3430 

5.270 

XB-235 

. .* 

198 

XB-236 

* 

187 

XB-237 

* 

501 

XB-238 

* 

535 

XB-239 

* 

4.050 

XB-240 

* 

4.000 

XB-241 

3800 

3.950 

XB-242 

3810 

6,170 

S-19 

3390 

678 

S-20 

3369 

275 

PND-108Lt 

3310 

400 

PND-108Rt 

3310 

400 

PND-118L 

3310 

430 

PND-118R 

3310 

430 

PND-104L 

3310 

260 

PND-104R 

3310 

260 

PND-125L 

3310 

193 

PND-125R 

3310 

255 


* P = 3400 is the value quoted by BTL for No. 2 material, 
t L and R refer to the left and right flake of a pair. 


Dynamic Characteristics of Thermistor Bolom¬ 
eters. The frequency response of the bolometers 
was measured beginning with a frequency of 1 cycle 
in a manner similar to that described in Section 
8.3.6 and under “Amplifiers,” Section 8.4. By means 
of a d-c amplifier, the d-c response was correlated 
with the response at some frequency greater than 
1 cycle. The frequency-response curves of the 
various BTL bolometers tested are shown in Figures 
36 to 39, inclusive. The height of the zero-frequency 
response is indicated by an arrow at the left edge. 
For purposes of comparison, there is shown in Fig¬ 
ure 34 the responses to be expected if the time 
constants (and sensitivities) listed in Table 9 were 
the sole factors determining the frequency responses 
of the S-19 and S-20 units. The usefulness of the 








































MEGOHMS MEGOHMS MEGOHMS 


BOLOMETERS 


/ 


257 





0.1 0.2 0.3 0.4 0.5 1 2 3 4 5 10 20 30 40 50 100 

MILLIWATTS 

Figure 34. Curves showing static characteristics of BTL bolometers. 






























































































































































































































258 


FAR INFRARED DETECTING ELEMENTS 


time constants quoted in Table 9, in predicting the 
frequency response, can be judged from these curves. 
The danger of incorrect extrapolation to zero fre¬ 
quency and the danger of using the zero-frequency 
sensitivity with the time constant to predict the 
behavior is clear from this figure. 



Figure 35. Exponential heating and cooling curves 
for unbacked BTL bolometer. 


However, BTL has found it convenient to think 
of the behavior of the solid-backed bolometers from 
the point of view that they possess several time 
constants. Figure 40 shows a curve in which sensi¬ 
tivity is plotted versus time of irradiation. It may 



FREQUENCY IN C 


Figure 36. Frequency-response curves for BTL bo¬ 
lometers. 

be seen that up to 10 milliseconds the data fit an 
exponential curve which has a definite steady-state 
sensitivity and a time constant. The experimental 
curve, however, keeps on rising as if it were tending 
to approach a second steady-state sensitivity at 
t — 1 second, then rises again, approaching a final 


steady-state sensitivity at 10 3 seconds. As a rough 
approximation, it could perhaps be said that the 
complete curve consists of the sum of a series ol 
exponential curves, each having its appropriate 
steady-state value and time constant. When a 
bolometer is used as a heat detector only the high- 
frequency portion of the frequency-response curve 
is of interest, as the exposure times rarely exceed 
two or three times the time constant associated with 
this portion of the curve. For each of the units S-19 
and S-20, the first time constant has the values 
listed in Table 9. 



Figure 37. Frequency-response curves for unbacked 
BTL bolometer. 


With the air-backed unit XB-108, however, the 
response shown in Figure 37 indicates that the zero- 
frequency sensitivity and a single time constant of 
135 milliseconds will serve to predict the complete 
frequency response almost exactly. A curve of sensi¬ 
tivity versus time of irradiation would obey an 
exponential rise to a steady state, as shown in 
Figure 35. 

The frequency-response curves of the other BTL 
units were measured and it was found that the pairs 
indicated by braces in Table 9 had nearly the 
same values. For this reason only the responses of 
one of each of the pairs has been included in Fig¬ 
ure 38. The typical response characteristics for air- 
backed, glass-backed, and quartz-backed units are 
easily distinguished. The time constants of the air- 
backed units are about as follows: XB-239, 73 
milliseconds; XB-240, 76 milliseconds; XB-241, 
122 milliseconds; and XB-242, 122 milliseconds. 

It was observed that the frequency responses were 
slightly different when the radiation was sharply 


14 I'iJli- limj l f.W 














BOLOMETERS 


259 



Figure 38. Frequency-response curves for BTL bo¬ 
lometers. 


focused on the bolometer than when diffusely spread. 
This might be expected because diffuse radiation 
would produce a general heating of the housing, 
backing, connections, etc. This is consistent with the 
observation that the frequency response was altered 
only in the 0- to 3-cycle region. Only a few decibels 
difference in response was noted for the variations 
in the incident radiation which were measured. The 
frequency responses shown in the figures were ob¬ 
tained with a sharply focused image of a Nernst 
glower on the bolometer strip. This should produce 
a bolometer heating more nearly like the electrical 
heating produced in the static tests and should give 
better correlation with these tests. 

The frequency-response curves of the two-element 
PND models are shown in Figure 39. The PND-104 
does not appear to have a time constant, but for 
frequencies greater than 1 cycle the response is 
roughly approximated by a 5- to 6-millisecond time- 
constant response. The two flakes in these PND 
models are mounted in close proximity on the same 
backing, and, because of this, the operation of one 
flake is not independent of the other. This can be 
observed when only one element of a pair is illumi¬ 
nated and the other element is used as a compen¬ 
sating resistor. Under this situation the ratio be¬ 
tween the zero-frequency response and the 1-cycle 
response is much less (about 10 db) than when an 
element of another bolometer is used as a compen¬ 
sating resistor. This indicates that the compensating 
flake in the PND is heated by conduction through 
the backing when the neighboring flake is irradi¬ 



Figure 39. Frequency-response curves for BTL bo¬ 
lometers. 


ated. The frequency response for values greater 
than 1 cycle was not materially affected by using 
an external compensating resistance in place of the 
neighboring flake in the PND models. 

The ENI values for the bolometers were deter¬ 
mined as outlined under Section 8.3.3 and under 
“Heat Sources,” Section 8.4. The noise from the 
amplifier (118 db gain from the grid of the first 



- 1 -,- 1 - 1 - 1 - 

FINAL STEADY-STATE SENSITIVITY 

8.6 VOLTS/WATTS PER VOLT BIAS 

- 

^ _ 


"*sf"FINAL STEADY-STATE FOR FIRST TIME CONSTANT - 

/ 

- / 



/ 

- 

_ 1 _ 

. . -1_1_1_1_1_ 


-3-2-1 O 1 2 3 

10 10 10 10 10 10 10 
TIME IN SECONDS 

Figure 40. Frequency-response curve for BTL bo¬ 
lometer. 

tube to the output meter) was about 0.1 volt for all 
of the bolometers tested. This noise was about 3 db 
above the expected Johnson noise from the input 
circuit. The ENI values were, therefore, propor¬ 
tional to the operating sensitivities. The smallest 
ENI values observed using the low-temperature 
black body described under “Heat Sources,” Sec¬ 
tion 8.4, are listed in Table 11. 

It should be emphasized that the ENI listed in 


ICT - fi r £~ 















260 


FAR INFRARED DETECTING ELEMENTS 


this table, as well as other ENI values in this 
chapter, are for the whole assembly of bolometer 
and amplifier. A change in the pass-band width 
would, of course, change the ENI. 


Table 11. ENI values for BTL bolometers. 


Bolometer 

ENI (jaw) 
observed 

Bolometer 

voltage 

Bolometer 
voltage 
for max 
sensitivity 

XB-108 

0.008 

75 

86.5 

S-19 

0.004 

202 

242.0 

S-20 

0.009 

208 

366.0 


A practical estimate of the merit of the heat 
detection system may be formed from the record 
shown in Figure 9. This record was obtained on an 
Esterline-Angus recording milliammeter as sug¬ 
gested in Section 8.4. The rectifier in this case, 
however, was built into the BTL amplifier. 

Sensitivity. Bell Telephone Laboratories Con¬ 
tract OEMsr-636 has used the symbol S b ] ^ c v 
(Table 9) to mean the rms voltage output of the 
bolometer bridge network per peak-to-peak watt 
of radiation falling on the bolometer under the 
conditions that the voltage across the bolometer be 
100 volts; that the heat input be interrupted 15 
times per second with equal times on and off; that 
the amplifier pass band be wide enough to preserve 
the essential waveform of the signal delivered by 
the bolometer bridge network to the amplifier; and 
that the bridge factor must be unity. 

A different definition of sensitivity which is con¬ 
venient to use here is denoted by the symbol s] \ 7 . 
This is defined as the rms voltage across the bolom¬ 
eter per rms watt of heat radiation falling on it, 
under the conditions that the heat input be modu¬ 
lated sinusoidally at / cycles, the bridge factor be 
unity, and the d-c bolometer voltage be V. Since the 
output of the bolometers to a 15-cycle square-wave 
heat input is essentially of triangular waveform, 
the above definition of sensitivity is related to the 
BTL definition by the equation: 

(2.22)S 6 ]^ v = «]«;. 

This equation will serve to correlate the values of 
Table 9 and the theoretical and experimental values 
which were obtained under Contract OEMsr-1168. 
The sensitivity of s] f v is nearly proportional to V. 
Actually, s](. is proportional to V/T 2 , but as the 


temperature of the bolometers with /? = 3400 de¬ 
grees C rises only from 298 (ambient) to 320.1 K 
for maximum sensitivity, the sensitivity may be 
taken to be nearly proportional to the bolometer 
voltage V. T and V are not both independent vari¬ 
ables after the ambient temperature has been fixed. 
If this proportionality is made use of, an approxi¬ 
mate sensitivity for 100 volts can be computed from 
the experimental value obtained for any other 
voltage. This relation has been verified experi¬ 
mentally. 

In order to compute the performance to be ex¬ 
pected from a bolometer, it is necessary to know 
either its voltage or current. Because the thermistor 
bolometer is used in a high-impedance bridge cir¬ 
cuit, more care and better equipment is required to 
measure the voltage than the current, so in the 
present investigation, currents were measured. With 
this current, the operating bolometer resistance and 
temperature were obtained from the R-T curves 
by a graphical construction as shown for unit S-19 
in Figure 33. A straight line, determined by the 
relation T — T 0 ( dT/dP)I 2 R , where T 0 is the 

ambient temperature and I is the bolometer current, 
was drawn. The intersection of this line with the 
R — T curve is the operating point (R and T ) of 
the bolometer. 

If it is desired to obtain the operating point when 
the bolometer voltage is given, the relation 
R(T — T 0 ) = ( dT/dP)V 2 may be used. The com¬ 
plete hyperbola need not be obtained. Two points 
on the hyperbola close to the R-T curve, prefer¬ 
ably on opposite sides, may be used with a linear 
interpolation between these points. This method is 
also illustrated for S-19 in Figure 33. 

Because the XB-108 unbacked unit cannot be 
used with a potential as high as 100 volts (cf. 
Figure 34), and because the S-19 and S-20 backed 
units can be used with operating potentials greater 
than 100 volts, the sensitivity does not tell 

the whole story. This sensitivity is not the maximum 
usable sensitivity, but the latter may easily be pre¬ 
dicted by the following relation derived in succeed¬ 
ing paragraphs. 

Max s f = (R 0 dT/dP) 1/2 a<f> f (0.105) volts per watt, 

where R 0 is the bolometer resistance at T 0 — 25 C, 
a is the absorptivity factor, <f> f is the factor account¬ 
ing for the smaller sensitivity at / cycles per second 
than at 0 cycles per second, and (0.105) is a con- 











BOLOMETERS 


/ 


261 


stant depending only upon /? = 3400 C and T 0 
= 25 C. The values of R, T, I, and V at this oper¬ 
ating point are 

R = R 0 (0.455) ohms 
T- T 0 + (22.1) C, 

I = (R 0 dT/dP)~ v * (6.98) amperes, 

V= (RodT/dP)-K (3.17) volts, 

where the bracketed numbers depend only on /? and 
T 0 . The maximum sensitivity was determined for 
the bolometers for the region of 5 p to 14 p, the 
values being listed in Table 12. While the maxi¬ 
mum sensitivity may not be used in practice, the 
useful sensitivity is proportional to it. If the 
“burn-out” temperature be the same for all bolom¬ 
eters of the same size and material, then whatever 
fraction of max s] f is safe for one bolometer should 
be safe for the others because this fraction is 
attained with the same temperature rise. The 
sensitivities of the units tested under OEMsr-1168 
are given in Table 12. 


Table 12. Sensitivity values for BTL bolometers with 
radiation in 5-n to 14-(ii region. 


Unit 

SA xoo 
OEMsr-636 
(v/w) 

2.22 SA loo 
OEMsr-636 
(v/w) 

i 15 

Si lOO 

OEMsr-1168 

(v/w) 

maxs] 15 

(v/w) 

XB-108 

234 

520 

372 

270 

S-19 

156 

346 

350 

675 

S-20 

66.5 

148 

154 

450 


Spectral Characteristics. The spectral character¬ 
istics of the bolometers were obtained by using 
them as the receiving elements in a Hilger infrared 



WAVELENGTH IN MICRONS 


Figure 41. Spectral response curve for BTL bolom¬ 
eter. 

spectrometer with a Nernst glower source. The lin¬ 
early amplified outputs of the bolometers were 
measured as a function of the wavelength of the 
energy passed by the spectrometer. These outputs 


were compared with similarly obtained outputs as 
measured with a Coblentz thermopile. The curves 
in Figure 41 are the ratios of the bolometer response 
to the Coblentz thermopile response. 

In order to have a more detailed picture of the 
spectral response, the spectrometer was made auto¬ 
matically recording by coupling the prism drive of 
the spectrometer to the chart drive of an Esterline- 
Angus recorder. The records obtained are shown in 
Figure 42. The four sections of each curve were 
made by varying the slit widths and amplifier gain 
so that the recorded region would produce a con¬ 
veniently measurable response. The records show 
regions of poor response at 3 p, 5 p, 7.5 p, 9 p, and 
11 p. The 6.5-p region is complicated by H 2 0 ab¬ 
sorption, but some of the low responsivity was 
undoubtedly attributable to the bolometer assembly 
and some possibly to the window coatings. 

It might be supposed from the results shown in 
Figure 41 that the bolometers transmit an apprecia¬ 
ble portion of the 5-p to 14-p radiation and that the 
quartz and glass backings reflect the transmitted 
portion back into the bolometers. This could ex¬ 
plain the better absorptivity of backed than un¬ 
backed bolometers in the 5-p to 14-p region. 

Experiments, reported by Pfund, 14 on the trans¬ 
mission of thermistor material in the infrared show 
that the transmission increases gradually from zero 
at about 2.5 p to about 17 per cent at 6.5 p. The 
measurements extend to about 10.25 p, at which the 
transmission has decreased to about 13 per cent. 

Absorptivity. The output from the bolometer and 
amplifier for any heat signal can be computed from 
a knowledge of the static characteristics of the 
bolometer, the input circuit and amplifier character¬ 
istics, and the frequency response of the bolometer 
unit. In addition, it is necessary to know what frac¬ 
tion of the radiant power incident upon the bolom¬ 
eter is utilized in changing its temperature. This 
fraction is called the absorptivity, and its value has 
been obtained by taking the ratio of the measured 
output from a heat signal to the output computed 
from the measured bolometer characteristics on the 
assumption that all the radiant heat power incident 
on the bolometer assembly was absorbed by it. 
When this factor is included in the calculation of 
the expected output, the calculated and measured 
outputs are obviously identical. The only test of the 
merit of the measurements, then, is the reasonable¬ 
ness of the absorptivity factor. 


RTCTHTfiTED 













262 


FAR INFRARED DETECTING ELEMENTS 




Figure 42. Records for determination of spectral-response curves for BTL bolometers. 


The absorptivity was evaluated for each bolom¬ 
eter for two kinds of radiation: first, the radiation 
from a standard lamp source with its peak near 2 p, 
and second, the radiation from a black body ranging 
up to 70 C above ambient temperature. The latter 
gives a wide band of energy, chiefly in the 5-p to 
14-p region. The absorptivities of the bolometers 
are listed in Table 13. These values are consistent 
with the curves shown in Figure 41 and discussed 
previously in this section under “Spectral Charac¬ 
teristics.’’ If the absorptivity curve is adjusted to 
the value computed for radiation from the standard 
lamp with a peak near 2 p, the value predicted by 
the curve for radiation with its maximum in the 
5-p to 14-p interval agrees with the measured value. 


Table 13. Absorptivity values for BTL bolometers. 


Bolometer 

Standard lamp 
max near 2 |a 

Black body 
max between 

5 |ii and 14 (.i 

XB-108 

0.72 

0.23 

S-19 

0.64 

0.53 

S-20 

0.87 

0.66 


Discussion of Equations Pertaining 
to Thermistor Bolometers 


The following paragraphs 15 are devbted to the 
derivation of relations used in connection with 
thermistors in this chapter and to showing how the 
ENI is affected by the bolometer characteristics. 

The voltage output per watt of incident radia¬ 
tion at a given frequency was given in equation 
(11), Section 8.3.6, for bolometers in general. This 
relation requires some amplification in the case of 
thermistor bolometers. The voltage into the ampli¬ 
fier per watt of incident radiation is given by 


jdRdT 
dT dP 


acpFW , 


(13) 


where s = volts into amplifier per watt of incident 
radiation; 


I = bolometer current; 

R = bolometer resistance; 

T = bolometer temperature (degrees K); 

P =r power (watts) causing the bolometer 
temperature rise; 


" W ES T mCTE fi- 















































































































































































































































































BOLOMETERS 


263 


a = absorptivity; 

F — bridge factor = R a /{R + Ra) (see Fig¬ 
ure 32); 

qp = frequency factor = output at operating 
frequency per output at zero cycles; 

W = waveform factor. 

Equation (13) may be rewritten with the aid of 
the thermistor resistance relation (14) 

B = Aexp-£ = .Roexp/j|l_-tJ (14) 

to read 

S = IR 4~z % a<fFW = V T* % a(fFW ’ (15) 

in which a negative sign has been omitted because 
the phase relation between output and input is not 
considered. The temperature rise of the bolometer 
above ambient temperature T 0 is given by 

(T - Tp) = AT = '-^pRl 2 = 1 t (16) 

Substituting (14) and (16) in (15) yields 

K„dT \' 4 
dP I 

[{ ^ Arexp/?q/r - my j n<pFTr . (17) 

The bracketed quantity depends only on A T, p, and 
T 0 and can easily be maximized. The bracketed 
expression will be maximum when 

3AT 2 + (2T 0 + p) AT — T 0 2 = 0. (18) 

As the fraction 12T 0 2 /(2T 0 + ft) 2 is small com¬ 
pared with unity 

AT = ——-for maximum s. (19) 

P -f- 2T 0 

For T 0 = 25 C and ft = 3400 (for material No. 2), 
which is assumed here for all numerical calcula¬ 
tions and emphasized by placing the numerical 
values in brackets, 

AT = [22.1 C]. 

The bracket in equation (17) equals 0.105 for the 
following values of the terms: p — 3400 K, T 0 
= 25C, AT = 22.1 C. Hence the maximum value 
of s is 


Max* = [0.i05] acpFW (20) 

and 

Max s]f = [0.105] 0<P/- (21) 

Max s]f is computed for F = 1 and W — 1. Using 
the value of AT for max s, we find that 

R (for max s) = R 0 [0.455] ohms, 

V (for max s) = | w/dP ) ^ 3 ' 17 ^ V ° ltS ’ ^ 

I (for max s) = | j [6.98] amperes. 

These relations specify the operating point com¬ 
pletely in terms of the bolometer resistance and the 
effect of the backing on the d-c heat dissipation, 
dT/dP. The sensitivity also depends upon the back¬ 
ing, because the latter determines the frequency 
factor qp and may affect the absorptivity a. For 
a well-blacked bolometer the absorptivity would 
not be affected by the backing and the merit of 
the backing could be evaluated by the factor 
(dT/dP) %q>: 

It is also interesting to note that if a group of 
thermistor bolometers of the same material and 
same flake size are inserted into the same input cir¬ 
cuit, then the bridge factor will be the same for all 
the bolometers when the supply voltage is adjusted 
for maximum sensitivity. This condition follows 
obviously from the fact that the operating resist¬ 
ances will all be the same regardless of the backing. 

Figure 34 shows that the bolometer voltage can¬ 
not be increased indefinitely. Experimental exten¬ 
sion of the curves shows that the above maximum 
voltage dV/dl becomes negative. This negative 
dV/dl region is an “unstable” one which allows the 
bolometer temperature to increase to the burning- 
out point if the attempt is made to increase V above 
its maximum value. This may be avoided by a bal¬ 
last resistor placed in series. 

If the only noise into the input of the amplifier is 
considered to be due to thermal agitation, the ENI 
to be expected from a bolometer and circuit may be 
computed from equations (17) and (23). 

Noise voltage = 1.22 X 10" 10 (iUA/) % ; (23) 

where 

R f = RF . 















264 


FAR INFRARED DETECTING ELEMENTS 


R' is the resistance of the input circuit as seen from 
the amplifier, and A/ is the proper pass band width. 
The result is 

TP A TT 1-22 X 10- 1Q (A//F)y*r 2 (24) 

a<t>(dT/dP)V*Wp\TK ’ 

The temperature function (T 2 /AT % ) is determined 
by the operating point and has a minimum value 
of (1.58 X 10 4 ) when T = 99.4 C with T 0 — 25 C. 
Since this AT is excessive, it would be better to oper¬ 
ate with the AT determined by a maximum sensi¬ 
tivity. For T = 20 C, (T 2 /AT%) is (2.26 X 10 4 ) 
and decreases slowly to the minimum at AT ^ 100 
degrees. For No. 1 material, ft = 3900 K, and AT, 
for maximum s, is 19.7 C. It is seen, therefore, that 
for bolometers with /? = 3900 K, the minimum 
equivalent noise input will not be much smaller 
(3 db at most) than the value of the ENI given by 
equation (24) for the operating point which gives 
maximum sensitivity. The minimum equivalent 
noise input is, therefore, given practically by 


vr^TT_1.22Xl0 10 (A//FTF) 1 /f T 2 1 
MENI_ a&idT/dP)* 


(25) 


with T = T 0 2 /(/5 -f- 2 T 0 ). The bracketed quantity 
is a function of ($ and T 0 only. 

Equation (25) gives a good picture of the way in 
which the bolometer (and amplifier) characteristics 
affect the MENI. It is seen from (25) that the re¬ 
sistance of the bolometer is contained only in the 
factor F, hence it affects the MENI only if it 
should be such as to make the input circuit values 
impractical for F approaching unity or if it should 
impair the design of an amplifier which reaches the 
noise limit set by equation (23). Equation (25) sep¬ 
arates the amplifier circuit design characteristics 
(A//FTF) 1/z , the bolometer material characteristics 
[T 2 //?AT 1/2 ], and the absorptivity, backing, and flake 
size characteristics, a<f>(dT/dP) 1/2 . 

The values of MENI obtained from equation (25) 
for the bolometers operating in the 5-p to 14-p re¬ 
gion in the input circuit shown in Figure 32 are 
given in Table 14, along with other properties of 
these 3 instruments. These were computed to be a 
measure of the d-c heat power before chopping into 
a 15-cycle square wave. The measured ENI values 
were not obtained with the voltage adjusted for 
maximum sensitivity, but are representative of the 
values obtained at lower sensitivities. Using equa¬ 
tion (24) to obtain the value of the ENI under the 


operating conditions for which the measured value 
of S-19 was obtained, it is found that the ENI is 
0.002 pw. This is in fair agreement with the 0.004- 
pw experimental value. 

Table 14. Collected characteristics of BTL bolometers. 



S-19 

S-20 

XB-108 

Backing * 

Glass 

Quartz 

Air 

Area,* cm 2 

6.02 X10" 3 

6.25 X10- 3 

5.79 X10" 3 

Ro megohms, 25 C 

3.93 

3.68 

3.92 

|3, degrees K 

3390 

3369 

3430 

dT/dP, degrees C/watt 

678 

275 

5270 

t milliseconds 

5.9* 

3.1* 

135 

(fls/i ) 

0.64 

0.87 

0.72 

#5^-15 ii 

0.53 

0.66 

0.23 

s] 15c 

J lOO v 

350 

154 

372 

Maxs] 15c 

ENI, jaw, d-c, chopped, 

675 

450 

270 

15 c (measured) 

0.004 

0.009 

0.008 

V for above ENI 

202 

208 

75 

V for max s 

242 

366 

86.5 

0,15 c 

MENI, 15 c [eq. (8)1 

0.257 

0.226 

0.079 

pw 

0.0025 

0.0033 

0.0072 

F 

0.111 

0.098 

0.117 


* Data furnished by BTL from Table 13. 


The expression for the final steady-state sensi¬ 
tivity has been quite rigorously derived in the 
appendix of the final report 12 on thermistor bolom¬ 
eter development under Contract OEMsr-636. It 
has the form 


s = v > a » FWaM volts per watt, (26) 

where s— final steady-state sensitivity; 

V b = voltage across the bolometer; 
a b =-p/T b 2 ; 

F — bridge factor = Rs/{R b + R 8 ) where 
R s — balancing resistor; 

W — waveform factor; 
a = absorptivity; 

C =heat dissipation constant; 

M — factor which takes into account the 
change in resistance of the balancing re¬ 
sistance in the bridge if it is an equal 
thermistor, or the effect of the balancing 
resistor if it is a metal. 


The factor M takes a different form for each of the 
two cases: 

1. If the balancing resistor R s is of thermistor 
material and is identical with the bolometer, the 

















OTHER BOLOMETERS TESTED BY SECTION 16.4 


265 


unbalance of the bridge circuit will cause changes in 
temperature, resistance, and current in this resistor. 
For this case 


M — 


1 

l-a b (T b -T 0 )' 


(27) 


2. If the balancing resistor R s is not necessarily 
equal to R b and is of metal and has a 8 — 0, the cur¬ 
rent and resistance changes produce an M factor of 
the following form: 

M =- l -(28) 

1 , m rr \ “ b 

Clft { 1 b I o ) | 


Equation (15) reduces to equation (26) for the 
final steady-state sensitivity by putting <f>, the fre¬ 
quency factor, and W, the waveform factor, equal 
to unity. Equation (15) is for the simplified case 
which presumes M = 1. This approximation is in 
most cases sufficiently good. 

Figure 43 shows a plot of equation (26), with M 
for the case of the thermistor balancing resistor, 
with sensitivity as ordinates, and V b) the bolometer 
voltage, as abscissa. Values of T b — T 0 are listed on 
the solid curve. Figure 43 also shows a dashed curve 
which is computed on the simplifying assumption 
that M is unity. The reason this curve deviates from 
a straight line is that a b decreases as T b increases. 


The same equation plotted with M for the condi¬ 
tion of a metallic fesistor is shown in Figure 44. 
Four curves are shown for various values of R s /R b . 
When this ratio is very small, the steady-state sen¬ 
sitivity rises very rapidly as V b increases to V p , the 
peak value, and continues to rise beyond the value 
corresponding to the peak voltage even though V b 
decreases again. 

It is also shown in the appendix of the report on 
thermistor bolometer development 12 that the expres¬ 
sion for the time constant, t, of the bolometer con¬ 
tains the M factor as a multiplier, and that 

t = %-M, (29) 

Lb 

where H — effective heat capacity of bolometer; 
C b — heat dissipation constant; 

M rvalues given by equations (27) or (28). 

88 OTHER BOLOMETERS TESTED 
BY SECTION 16.4 

8,8,1 The Felix Bolometer 

Four bolometers, numbers 2A, 11 A, 15, and 19, 
developed by the Heat Research Laboratory of the 
Massachusetts Institute of Technology under Con- 


3.0 xio 3 


2.5 


1.0 


0.5 


1 1 1 1 1—1—1 1 1— 

_ R s IS A RESISTANCE WITH 

CHARACTERISTICS IDENTICAL WITH 

THE BOLOMETER ON WHICH THE 

1—1—1—1— 

-[ 

— 

RADIATION 

- 

C =0.0025 WATTS/C 
Rbo s 4x10*0HMS 

IS INCIDENT 

i 

/ ' 

/ 

-•'•i 

> 

T 0 =300K 
— >S = 3400 K 

zc 

A* 

r 

X 

N 

\ 

X 

— 

INI 

ITIAL SLOPED. 

£ 

-"6* ~X'‘° '5' 

X 

20% 
25 • 
30 8 

— 



^X 

60l J 

70 

35x 

40x / 

V 

— 

x* 

i_i_ 

1111_ 

90 8 °^ X 

<00, x - X 

- 

_1_1_1_1_ 

_L 

— 


<00 


200 


300 v c 


W IN VOLTS 


Figure 43. Graph showing steady-state sensitivity of BTL bolometer plotted as 
function of Vb where compensating resistor is another thermistor. 

























266 


FAR INFRARED DETECTING ELEMENTS 



Figure 44. Graph showing steady-state sensitivity of BTL bolometer plotted as func¬ 
tion of V* where compensating resistor is a metallic one. 


tract NDCrc-180, were sent to Ohio State Univer¬ 
sity under Contract OEMsr-1168 for test purposes. 
These bolometers, of identical design, are composed 
of four blackened nickel strips connected in series 
and mounted close together and parallel in a small 
cylindrical holder. The electrical leads are sealed 
into the back slightly above the diameter; the front 
is covered with a hemispherical silver chloride win¬ 
dow. The whole unit is about lxl centimeter and is 
sealed with a filling of hydrogen at a pressure of 
4 millimeters of mercury. The bolometers have a 
total resistance of about 16 ohms and are operated 
at 1.7 volts across the whole unit. While the ele¬ 
ments are rectangular strips, they are masked so 
that the receiver area of 0.172 square centimeter is 
circular. The blacking material on No. 2A and No. 
15 is aluminum black; on No. 11A and No. 19 it is 
gold black. 

Static Characteristics. The resistance of the bo¬ 
lometers, measured with a Wheatstone bridge while 
the temperature was varied from 50 to 25 degrees C, 
was linear, and, from the slope of the curve, the 
temperature coefficient of resistance a was obtained. 
The values of a are listed in Table 15. 

The variation of the bolometer resistance R b with 


d-c electric power input to the bolometer was meas¬ 
ured with the Wheatstone bridge circuit, the power 
into the bolometer being varied by varying the volt¬ 
age supply to the bridge. Curves showing the varia¬ 
tions of R b with E b , the voltage across the bolom¬ 
eter, with I b , the current through the bolometer, and 
with P, the electric power expended in the bolom¬ 
eter, may be found in Figures 13 and 14 of OSRD 
Report 5992. 2 The slopes, dR b /dP, for the bolom¬ 
eters are listed in Table 15. 

As in the case of the Strong bolometer, not all the 
heat power is effective in producing resistance 
change, nor are all wavelengths necessarily equally 
effective. Direct measurements (cf. Section 8.4) 
were, therefore, made to determine the change in re¬ 
sistance of the strip per watt of incident heat power, 
dR b /dH , of a known wavelength. The absorptivity, 
a x , was then obtained as the ratio ( dR b /dH)/ 
( dR b /dP ). The values of dR b /dH and a for the 
bolometers are tabulated in Table 15. 

The spectral response was obtained from 1.0 p to 
14.0 p for all bolometers. Curves 1 and 2 in Figure 
45 show the response, relative to the response of the 
Coblentz thermopile, of the No. 11A unit adjusted 
to an absorptivity of 0.42 at 2.0 p. Curve 1 was 





























OTHER BOLOMETERS TESTED BY SECTION 16.4 267 



Table 15. Values of 

various constants for the Felix bolometers. 


Constant 

Units 

No. 2A 
(A1 black) 

No. 15 
(A1 black) 

No. 11A 
(Au black) 

No. 19 
(Au black) 

a 

Per degrees C 

0.00366 

0.00299 

0.00330 

0.00396 

dRb/dH 

Ohms per watt 

10.3 

13.3 

10.7 

21.2 

dRb/dP 

Ohms per watt 

16.25 

15.5 

25.4 

28.2 

dRb/dT 

Ohms per degree C 

0.0384 

0.040 

0.0341 

0.046 

dT/dP 

Degrees C per watt 

424. 

388. 

732. 

613. 

0.2 fl 

Absorptivity 

0.64 

0.S5 

0.42 

0.75 

Oofi-Hfi 

Absorptivity 

0.64 

0.45 

0.15 

0.65 

Center-strips 

Milliseconds 

20.2 

21.4 

21.8 

19.0 

ENI (14.6 c) 

p.w, 5 p, to 14 p, 

0.02 

0.03 

0.05 

0.01 

ENI (27.6 c) 

pw, 5 p, to 14 p, 

0.04 

0.03 

0.11 

0.02 

MENI (14.6 c) 

pAv, 5 p, to 14 p, 

0.002 

0.003 

0.005 

0.001 

MENI (27.6 c) 
Minimum detectable 

pw, 5 p, to 14 p, 

0.005 

0.01 

0.02 

0.004 

signal (27.6 c) 

pw, 5 p, to 14 |ii 

0.04 

0.10 

0.12 

0.015 


obtained when one of the edge elements was illu¬ 
minated and curve 2 when one of the central ele¬ 
ments was illuminated, indicating that the response 
is better by a factor of about 2 from 6 p to 14 p for 



Figure 45. Absorptivity curves for Felix bolometers. 

edge-strip than for central-strip illumination. Curve 
4 in Figure 45 shows the relative spectral response 
of bolometer No. 2A with respect to the Coblentz 
thermopile throughout the 1.0-p to 14.0-p region. 
The absorptivity of No. 2A is approximately 0.64 


throughout, and the curve is adjusted to this value 
at 2.0 p. 

Bolometer No. 2A was exposed to sunlight, and 
the silver chloride window became blackened to 
visible light. After this had occurred, its spectral 
response was again determined from 1.0 p to 14.0 p, 
and curve 3 in Figure 45 shows the response rela¬ 
tive to the Coblentz thermopile after it had been 



WAVELENGTH IN MICRONS 


Figure 46. Absorptivity curves for Felix bolometers. 

exposed. The curve is adjusted to an absorptivity 
of 0.15 at 2.0 p. It may be seen from curves 3 and 
4 that from 1 p to about 7 p the absorptivity is less 
than 50 per cent of the value before it was exposed. 
In the atmospheric window region, 8 p to 14 p, the 
absorptivity, though smaller than in the unexposed 
state, is everywhere more than 50 per cent of the 
original absorptivity. 

Figure 46 shows the absorptivity curves for bo¬ 
lometers No. 15 and No. 19 adjusted to the respec- 











268 


FAR INFRARED DETECTING ELEMENTS 



FREQUENCY IN C 

Figure 47. Frequency-response curves for Felix bolometers. 


tive values 0.86 and 0.75 at 2.0 jx, at which wave¬ 
length a direct measurement was made. 

Dynamic Characteristics. The frequency response 
and time constant for each bolometer were deter¬ 
mined from 1 to 100 c. The experimental curves for 



Figure 48. Frequency-response curves for Felix bo¬ 
lometer. 


all the bolometers approximate very closely the 
type of curve to be expected if a time constant for 
the bolometer exists. Figures 47 and 48 show the 
frequency-response curves for the Felix bolometers. 
The time constants are listed in Table 15. 


It was observed, however, that each of the four 
strips of the Felix bolometer appeared to possess 
its own individual time constant. Table VI of 
OSRD Report 5992 2 lists the values of t, deter¬ 
mined by the parallelogram method, for each strip 
of all the bolometers. 

The ENI and MENI for these bolometers were 
determined at the frequencies 14.6 and 27.6 c, and 
Table 15 lists the values found. 

By the method described earlier, the minimum de¬ 
tectable signal [MDS] (see Section 8.3.4) for the 
bolometers was obtained. A record showing both 
signal and noise on the same chart for signals above 
the noise is shown in Figure 7. The values of the 
MDS determined from such charts are found in 
Table 15. 

From the static characteristics and frequency re¬ 
sponse of the bolometers and the amplifier gain 
characteristics, the output voltage per watt of heat 
power input was calculated. These predicted values 
were compared with the observed outputs and the 
results of the comparison are listed in Table 16. 
The largest difference between observed and pre¬ 
dicted values is less than 2 db and in most cases is 
about 0.5 db. This is good agreement. The agree¬ 
ment is considerably better than in the case of the 


w \ j x UL/ 













OTHER BOLOMETERS TESTED BY SECTION 16.4 


269 


Strong bolometer. This can probably be explained 
by improvements in measuring technique made dur¬ 
ing the interval between the two sets of measure¬ 
ments. It was not possible to remeasure the Strong 
bolometer with the improved technique, because an 
element had burned out. 


Table 16. Table showing the expected output com¬ 
pared with the measured output for radiation in the 
5-n to 14-p, region. 



No. 

2A 

No. 

15 

No. 

11A 

No. 

19 

Volts/pw observed, 14.6 c 

19.0 

11.4 

4.95 

25.9 

Volts/pw expected, 14.6 c 

18.5 

12.2 

6.13 

26.3 

Difference (db) 

0.2 

0.52 

1.8 

0.2 

Volts/nw observed, 27.6 c 

6.1 

4.05 

1.8 

9.4 

Volts/pw expected, 27.6 c 

5.87 

3.8 

1.9 

8.5 

Difference (db) 

0.3 

0.52 

0.42 

0.85 


882 The Polaroid Evaporated Metal-Strip 
Bolometer 

A number of evaporated nickel bolometers made 
by the Polaroid Corporation Laboratories under 
contract with Division 5, NDRC, were submitted 
for testing. One of these, an evaporated nickel strip, 
was constructed for Wright Field and designated 
WF-1. Its unit was about 1x0.5 millimeter, 0.02 p 
thick, and was evaporated onto a nitrocellulose film 
about 0.06 p to 0.08 p thick. The resistance of the 
bolometer was about 15 ohms; its receiving area 
had a coating of antimony black. Three other bo¬ 
lometers (Nos. Ni324, Ni347, and Ni350) were 
evaporated onto a nitrocellulose film, in the form 
of a cross with each leg having an area of about 
0.045 square centimeter. Each bolometer was 
mounted in a cylindrical brass case about 2 centi¬ 
meters outside diameter and 1 centimeter long, with 
a silver chloride window covered with a protective 
coating. Table 17 lists the resistances of Nos. Ni324, 
Ni347, and Ni350. All four bolometers operate in 
air at atmospheric pressure. 

Static Characteristics. The variation of the bo¬ 
lometer resistance with temperature and with elec¬ 
tric power expended was measured by means of a 
Wheatstone bridge. The resistance variation of one 
leg when the power into an adjacent leg varied was 
also measured. Heating one strip obviously pro¬ 
duces a measurable temperature rise in the others. 

Dynamic Characteristics. The frequency-response 


characteristics of the bolometers were measured 
from 1 to 400 c and are shown in Figure 49. Only 
WF-1 had a time constant of value 4.7 milliseconds. 
The other three frequency-response curves did not 



Figure 49. Frequency-response curves for Polaroid 
bolometers. 


warrant a computation of the time constant. Legs 
A and C of No. Ni350 were identical in response, so 
characteristics of only leg A of Nos. Ni347 and 
Ni324 were measured. All frequency-response 
curves have about the same shape. 


Table 17. Static characteristics for the Polaroid 
bolometers. 





a = 30 C 



dR/dP 

dR/dT 

(per 

Bolom¬ 


(ohms/ 

R (ohms per 

degree 

eter 

Leg 

watt) 

(ohms) degree C) 

C) 

No. Ni324 

A 

119 

63.6 0.217 

0.00168 


C 

118 

65.0 



dR leg A 

dP leg C 

= approximately 10 ohms/watt 


No. Ni347 

A 

130 

43.2 



B 

132 

44.0 0.154 

0.00175 


C 

130 

43.6 



D 

127 

44.7 



dR leg 1 
dP leg 2 

= approximately 5 ohms/watt. 

It does 


not matter whether leg 1 and leg 2 are 



adjacent. 


No. Ni350 

A 

138 

132.4 



B 

148 

122.1 0.0334 

0.00104 


C 

153 

123.1 



D 

139 

133.0 



The ENI was measured for each bolometer, using 
a tuned amplifier working at 14.6 c with special 
input transformer furnished by Polaroid. The ENI 


















270 


FAR INFRARED DETECTING ELEMENTS 


values are listed in Table 18. Figure 8 shows an 
Esterline-Angus recorder trace of signal and noise 
on the same record for bolometer No. Ni324. The 
MDS is about 0.015 pw. No spectral response meas¬ 
urement was made on any of the Polaroid bolom¬ 
eters. 


Table 18. ENI values for the Polaroid bolometers 
determined with 5-p, to 14-p, radiation. 


Polaroid 


unit 

ENI 

324 

0.01 

347 

0.01 

350 

0.02 

WF-1 

0.02 


883 The Radio Corporation of America 
[RCA] Bolometer 

Two RCA bolometers, No. 18 and No. AX, were 
sent by the Bureau of Ships to Ohio State Univer¬ 
sity Contract OEMsr-1168 for testing. The elements 
are made of a tellurium-zinc alloy evaporated onto 
a nitrocellulose film which is mounted on a plastic 
base. The operation is in vacuum with a window of 
silver chloride closing the unit on the front face. 
The bolometer receiver is in the form of a strip 
0.5x1.0 millimeter having a resistance of about 
10,000 ohms and a negative temperature coefficient. 

26 C-*-U-OC--25.7 C-► -»26.1 C~ 


- 






- 

- 







- 

- 


_1_ 




-1- 

LA 


Figure 50. The effect of past history on the resist¬ 
ance of the RCA bolometer No. AX. 

Static Characteristics. Attempts to measure the 
temperature dependence of the resistance of these 
bolometers were not very successful, because the 
resistance appeared to depend also on the history of 
the samples. It was therefore not ascertained 


whether they obeyed the characteristic thermistor 
relation, R — R 0 exp (fl/T ). At 26 degrees the tem¬ 
perature coefficient is about 0.009 C for No. 18 and 
0.0075 C for No. AX. 

26C-*|*-O C-4*- 26c -•* 



HOURS 


Figure 51. The effect of past history on the resist¬ 
ance of the RCA bolometer No. 18. 

The nature of the effect of history upon the bo¬ 
lometer resistance is indicated in Figures 50 and 
51. In these figures the resistance has been plotted 
as a function of time, and the temperature of the 
bolometer is noted. The bolometer was quickly 
changed from room temperature to about 0 C and 
later changed back. A rapid temperature change 
appears to produce a larger resistance change than 
does a slow one. 



Figure 52. Frequency-response curve of the RCA 
bolometers. 


Dynamic Characteristics. The time constants for 
both bolometers were measured. The frequency-re¬ 
sponse curves, plotted in decibels versus log fre¬ 
quency, are shown in Figure 52. For clarity the curve 
for No. AX is displaced an arbitrary amount from 
that of No. 18. In the region available for measure¬ 
ment, the frequency-response characteristics of the 
units can apparently be represented by time con- 

































OTHER BOLOMETERS TESTED RY SECTION 16.4 


271 


stants but from Figures 50 and 51 it may be seen 
that these cannot be effective down to zero fre¬ 
quency. The No. AX unit also deviates slightly 
from true time-constant behavior above 20 c. The 
time constants are about 70 milliseconds and 80 
milliseconds for No. AX and No. 18 respectively. 



Figure 53. Auxiliary input stage used with RCA 
bolometer. 


The ENI values, measured by using the input 
circuit shown in Figure 53 followed by the tuned 
amplifier working at 14.6 c, are listed in Table 19. 
The noise output from the bolometers was several 
times greater with the bolometer current on than 
with it off. When no current flowed, the output 
noise arose in the first tube of the amplifier rather 
than in the bolometer input circuit. An increase in 
current, with a larger accompanying voltage, pro¬ 
duced an increase in noise voltage as well as in 
signal voltage. Bolometer voltages up to 5 or 6 
volts were tried but resulted in a poorer signal- 
to-noise ratio than when only 1 volt was applied. 

Table 19. ENI values for the RCA bolometers deter¬ 
mined for 5-p, to 14-p, radiation at 14.6 c. 


Circuit 1 Circuit 2 Circuit 3 

18 AX 18 AX 18 AX 


Amplifier output 


noise (v) 

0.15 

0.15 

0.8 

1.0 

0.8 

0.6 

ENI (fxw) 

Volts across 

0.02 

0.02 

0.045 

0.05 

0.04 

0.03 

bolometer 

1.2 

1.8 

3.7 

5.4 

4.3 

6.2 


Circuits 1, 2, and 3 referred to in Table 19 mean 
that in the input circuit of Figure 53 E — 7.5 volts, 
R = 50,000 ohms wire wound; E — 45 volts, R = 
100,000 ohms wire wound; and E = 45 volts, R = 
75,000 ohms, respectively. A metalized IRC series 
resistor was used in each instance. The amplifier 


output noise, found to be 0.07 Volt for all circuits, 
was determined by replacing the bolometer battery 
with a short circuit. Because of the difficulty of 
estimating the noise and because of the apparent 
variations in the bolometer characteristics, the ENI 
values are not so reliable as in the cases of the other 
types of bolometer. It appears also that the larger 
bolometer voltage with its greater working sensi¬ 
tivity is of no real advantage for the detection of 
small signals at 14.6 cycles. 

Using circuit 1 with the 14.6-cycle tuned ampli¬ 
fier, the value of the Johnson noise would be about 
12 db below the measured noise. The MENI is, 
therefore, about 0.005 pw. 

Another indication of the merit of the bolometer 
may be had from the record reproduced in Figure 
10, which shows the magnitude of the noise and of 
the noise plus signal for a small amount of 5-p to 
14-p radiation. With a minute of exposure, 0.03 pw 
may be readily detected. 

The spectral response was obtained and is shown 
in Figure 54. The ratio of the response to that of the 
Coblentz thermopile has arbitrarily been adjusted 
to unity at 2.0 p. 



WAVELENGTH IN MICRONS 


Figure 54. Spectral-response curve for RCA bolom¬ 
eter. 


884 The Superconducting Bolometer 

Two superconducting bolometers, No. 1 and No. 
4, built in the Johns Hopkins laboratories under 
the supervision of D. H. Andrews, were sent, 
through arrangements made with Section 16.4, to 
the Ohio State University Contract OEMsr-1168 
by the Bureau of Aeronautics for tests on sensi¬ 
tivity, frequency response, etc. 

The cryostats, used to control the operating tem¬ 
perature of the bolometers, are copper cylinders 
about 1 foot long by 6 inches in diameter, with the 

















272 


FAR INFRARED DETECTING ELEMENTS 


receiver at one end! The detecting element of this 
bolometer is a strip of columbium nitride with effec¬ 
tive dimensions 0.317x0.0254 centimeter, receiving 
area 0.00808 square centimeter. This strip is ce¬ 
mented to a thin sheet of bakelite which is fastened 
to a copper block in contact with the liquid hydro¬ 
gen chamber. A piece of polished rock salt, which 
transmits radiation from 1 p to 15 p, serves as a 
window. The window makes a vacuum seal with the 
outer container which must be evacuated before the 
cooling chambers are filled. From the end opposite 
the receiving element emerge the electrical connec¬ 
tions and the filling tubes. The cooling chambers 



Figure 55. Working temperature region of super¬ 
conducting bolometer. 


are concentric cylinders, the outer one being for 
liquid nitrogen, and the inner one, of 1.0 liter 
volume, for liquid hydrogen. A copper block, to 
which the receiving element is attached, makes good 
contact with the hydrogen chamber. Around this 
block is wrapped a heating coil of about 500 ohms, 
so that the operating temperature of the bolometer 
may be controlled. 

The bolometer becomes sensitive to small heat 
changes when its temperature is between 14.2 and 
14.3 K or slightly above the triple point of hydro¬ 
gen (see Figure 55). Good insulation to heat from 
outside the cryostat is provided by the vacuum in 
the outer container. Liquid nitrogen is forced under 
pressure from a Dewar flask into the cooling cham¬ 
ber until the nitrogen streams from the exit tube. 
The hydrogen chamber is then precooled by par¬ 


tially filling it with nitrogen, which is blown out 
before the hydrogen is admitted. 

The input circuit used is shown in Figure 56. The 
leads from the heater coil, the bolometer, and a 
dummy resistance emerge from the back of the 
cryostat and lead to the input circuit as shown. 
The complete input circuit was shielded in a metal 
chassis. Two potentiometers, Pi and P 2 , and switch, 
Si, provide control of the heater current. 


MILLIAMETER 



Figure 56. Input circuit for superconducting bolom¬ 
eter. 


Static Characteristics. No measurements were 
made on a, the temperature coefficient of resistance, 
or on dR/dP, the rate of change of R with electric 
power, or on the absorptivity. 



Figure 57. Frequency-response curve for supercon¬ 
ducting bolometer. 


Dynamic Characteristics. The frequency re¬ 
sponse of cryostats No. 1 and No. 4 was measured 
from 1 to 400 cycles and from 1 to 2,000 cycles 
respectively. 

Figure 57 shows the frequency response of bolom¬ 
eter No. 4. The curve is that which would be ex¬ 
pected if the instrument had a time constant of 1.4 





























OTHER BOLOMETERS TESTED BY SECTION 16.4 


/ 


273 


millisecond. As the frequency response for No. 1 is 
that which would be expected if it had a time con¬ 
stant of 1.8 millisecond, the curve is not shown. 
Although their physical sizes are comparable, the 
time constants of the two bolometers are smaller 
than the time constants of most of the other bolom¬ 
eters tested by a factor of about 10, and are smaller 
than the quoted time constants of the BTL ther¬ 
mistors by a factor of about 2. 

The ENI values were determined for both No. 1 
and No. 4 at 14.6 cycles and 27.6 cycles with a 
tuned amplifier, and at 20 cycles and 14.6 cycles, 
respectively, using a wide pass band amplifier. 
Table 20 gives the ENI values for both bolometers 
under the various conditions. 

Figure 11 shows an Esterline-Angus record made 
to determine the MDS at 27.6 cycles for the cryo¬ 
stat bolometers. For No. 1 the MDS was 0.0005 pw, 
and for No. 4 it was 0.0006 pw. 

The spectral response of the two was compared 
with the response of the Coblentz thermopile which 
has been used as a standard in previous measure¬ 


ments. In Figure 58 are given curves showing the 
relative response arbitrarily adjusted to unity at 
2.0 p where the glower has its peak. It may be seen 
that from 1 p to 6 p the responses are about the 
same, but from 6 p to 14 p these bolometers appear 
to become gradually blacker than they were at 2 p. 


Table 20. ENI values for the cryostat bolometers for 
5-p to 14-jn radiation. 


Bolom¬ 

ENI (jaw) 

ENI (jaw) 

ENI (jaw) 
wide-band 

eter 

14.6 c 

27.6 c 

amplifier 

1 

0.0006 

0.00035 

0.019 (20 c) 

4 

0.0015 

0.0006 

0.024 (14.6 c) 


The measurements reported above show that the 
cryostat bolometer is more sensitive by a factor of 
about 10 than any other device tested under Con¬ 
tract OEMsr-1168. It has, in addition to its high 
sensitivity, a small time constant and it appears to 
be quite black throughout the whole spectrum. 



WAVELENGTH IN MICRONS 

Figure 58. Spectral-response curve for superconducting bolometer. 








274 


FAR INFRARED DETECTING ELEMENTS 


885 The Donau Gerat Bolometer 

One Donau Gerat evaporated metal-strip bolom¬ 
eter was received from Section 16.4, NDRC, and 
two from the Engineer Board, Fort Belvoir, Vir¬ 
ginia, for testing. For easy reference they were 
numbered and described in the following manner: 

No. 1 in glass case equipped with window, rotat¬ 
ing shutter and cable; 65 ohms per strip. 

No. 2 in glass case equipped with window; 950 
ohms per strip, worthless as a heat detector. 

No. 3 without case or window; 90 ohms per strip, 
one strip defective. 

The bolometers are apparently made of metal, 
without backing, evaporated onto a thin film of 
nitrocellulose. There are two strips set parallel with 
each other, with all four end leads available. The 
nitrocellulose skin bearing the bolometers is 
stretched on a plastic frame across a hole in the 
center of the frame. The unit is fitted into a glass 
tube into which the four leads are sealed. The 
window is a circular plate of a mixture of thallous 
bromide and thallous iodide said to transmit radia¬ 
tion fairly well out to 40 p. As the leads are sealed 
into the glass and the window is sealed to the tube 
containing the bolometer, it is presumed that the 
unit operates at reduced pressure, perhaps with 
some gas other than air. 

Static Characteristics. The values of the static 
characteristics of the Donau Gerat bolometers, 
dR/dT, a, and dR/dP are grouped in Table 21. 


Table 21. Static characteristics of the Donau Gerat 
bolometers. 



dR/dT 


dR/dP 

Bolometer 

(ohms/degree C) 

a/degree C 

(ohms/watt) 

1 

0.116 

0.00195 

153 

3 

0.130 

0.00130 

155 


Dynamic Characteristics. Frequency-response 
curves, over the range 1 to 150 c, were found for 
bolometers No. 1 and No. 3, as shown in Figure 59. 
It may be seen that the curve for No. 1 approxi¬ 
mates that for a time constant of 10.9 milliseconds, 
while the curve for No. 3 is quite different. It should 
be recalled that No. 1 is enclosed and presumably 
operating at somewhat less than atmospheric pres¬ 
sure, while No. 3 is not enclosed. 

The ENI for bolometer No. 1, operated at 2.0 
volts in the tuned amplifier, was 0.051 pw peak to 


peak at 14.6 cycles and 0.113 pw peak to peak at 
27.6 c. 

The Johnson noise should have been smaller than 
the observed noise by 26 db for the 14.6-c measure¬ 
ment and 20 db for the 27.6-c measurement. If the 
amplifier noise and current noise were eliminated, 
the value of the MENI would be about 0.003 pw 
and 0.011 pw at the two frequencies respectively. 
No spectral-response curve was made. 



FREQUENCY IN C 


Figure 59. Frequency-response curves for Donau 

Gerat bolometer and Italian bolometer. 

886 The Italian Bolometer 

Two bolometers of Italian make were sent to 
Ohio State University through Section 16.4, NDRC. 
One was received broken; the present remarks con¬ 
cern only the other. The bolometer is a thin metal 
strip having a resistance of about 280 ohms, sup¬ 
ported on a nitrocellulose film, and enclosed in a 
brass case with a rock salt window. The case is 
apparently connected to one end of the bolometer, 
and the other lead emerges through a fiber insulator. 
In one end is soldered a tube which is presumably 
for evacuation, but the bolometer was not evacu¬ 
ated for the tests described. The metal strip had a 
very wrinkled appearance. Its linear dimensions are 
4x5 millimeters, giving an area of 0.2 square centi¬ 
meter. No information was furnished concerning its 
recommended operating voltage. 

Static Characteristics. The value of dR/dT was 
found to be about —0.35 ohm per degree C, and 
the value of dR/dP to be about —200 ohms per 
watt. It was found to be very difficult to make good 
resistance measurements on this bolometer, prob¬ 
ably because of the poor contacts provided by its 
crinkly surface. 


.* I; 'ii-t is 
UliO 1. IllCTI IYTX 










OTHER HEAT-SENSITIVE DEVICES 


275 


Dynamic Characteristics. The frequency response 
from about 2.5 to about 40 c is shown, as obtained 
with a General Motors amplifier, by the curve of 
Figure 59. No measurement of ENI was made as 
no signal above noise could be discerned with the 
largest available calibrated heat input (69 pw per 
square centimeter). 

89 OTHER HEAT-SENSITIVE DEVICES 
891 The Golay Heat Detector 10 

Three separate Golay units were investigated 
under OEMsr-1168. The first was a unit loaned by 
the Signal Corps Laboratories at Eatontown, New 
Jersey. The other two units were more recent mod¬ 
els built by Don Lee, Incorporated, for Naval 
Aeronautics Modification Unit (NAMU) at Johns- 
ville, Pennsylvania, and furnished for test by the 
Bureau of Aeronautics, Navy Department. These 
units will hereafter be referred to respectively as 
a, 1, and 2. 


PRISM 



The Golay heat detector is shown schematically 
in Figure 60. Its operation has been described in 
Section 8.2.3. The photocell circuit which is part of 
the unit is shown in Figure 61. A cathode-follower 
circuit was introduced between the photocell and 
the amplifier, which reduced the impedance of the 
photocell circuit to a 50,000-ohm output. This per¬ 
mits the use of a cable from the detecting device to 
the amplifier and also allows the detecting device 
to be at some distance from the signal observing 
post. 


thallous sulfide 
CELL 



Figure 61. Input circuit of Golay detector. 


The Golay device must always be operated with 
intermittent illumination; in fact, the construction 
is such that at zero frequency it gives zero response. 



FREQUENCY IN C 

Figure 62. Frequency-response curve of the Golay 
detector. 


Measurements on the frequency response of these 
instruments showed that in no case can a time con¬ 
stant be ascribed. The first unit (a) was found to 



Figure 63. Frequency-response curves of Golay de¬ 
tectors 1 and 2. 


give a maximum response when operated near 140 c. 
The frequency-response curve is shown in Figure 62. 
Corresponding curves for the later models (1 and 2) 
are shown in Figure 63 and indicate that maximum 










































276 


FAR INFRARED DETECTING ELEMENTS 


response would be obtained when operated near 

10 c. 

The ENI values were obtained for all three units, 
the first at a frequency of 140 c and the others at 
14.6 c. The values are given in Table 22. 


Table 22. ENI values for the Golay heat detectors 
determined for radiation in 5-n to 14-^ region. 


Bolom¬ 

eter 

ENI (14.6 c) 

ENI (140 c) 

Area of 
receiver 

a 


0.010 MAV 

0.01 sq cm 

1 

0.0025 (liw 


0.071 sq cm 

2 

0.0014 piw 


0.071 sq cm 


The spectral response of these detecting elements 
was investigated in the usual manner, and it was 
found that the receiving elements were uniformly 
black throughout the spectral region 1 \x to 14 ja. 

It may be seen from the foregoing that Golay 
detectors may be produced which have an extremely 
rapid response, but at the same time maintain very 
low ENI values. 

810 COMPARISON OF THE DETECTORS 
8,10,1 Basis of Comparison 

In the foregoing sections of this chapter the con¬ 
struction details and performance characteristics 
of the various detectors developed in Section 16.4 
and elsewhere and tested under Contract OEMsr- 
1168 have been discussed in detail. It is the purpose 
of this section to compare these characteristics so 
that some estimate of their relative merit may be 
had. Certain of these characteristics may be singled 
out to form the basis of this comparison. It is 
assumed that the primary interest in these detec¬ 
tors is in their military applications. As most mili¬ 
tary objectives are at a temperature which is 
different from the background by only a few de¬ 
grees, it is important that the detectors be capable 
of detecting exceedingly small amounts of radiant 
power above the noise background, that they have 
sufficiently good response at high frequencies to per¬ 
mit rapid scanning, and that their response to radia¬ 
tion be high at least in the window region (8 p to 
14 p). 

The quantities measured relating to the above 
requirements are the ENI, the frequency response, 
and the spectral response. It must be recalled that 
the ENI is dependent on a number of factors. 


Ideally the amplifier used with these detectors 
should be noiseless so that it should be possible to 
work down to the noise threshold of the detector 
itself. The lowest output value of the noise would 
be the amplified Johnson noise of the detector. It is 
supposed that there are no microphonic or current 
noises associated with the detector. It was found 
impossible in any instance to work down to the 
limit of the Johnson noise because of noises orig¬ 
inating in the amplifier. In some cases, current 
noises were quite significant. On the other hand 
microphonic noises, in general, contributed very 
little to the total noise output. It did not seem ex¬ 
pedient to attempt to build an amplifier with less 
noise than the one described under “Amplifiers,” 
Section 8.4. Except in cases where current noises 
were significant, the measured noise output of the 
tuned amplifier when the detectors were in the 
circuit was about the same for all detectors for all 
frequencies of operation. The measured ENI is, 
therefore, not as good a method of comparison as 
it could have been if it had been possible to work 
down to the Johnson noise. 

From the time-constant or the frequency-response 
curve, it can be seen how the response changes with 
the frequency of interruption of the radiation. One 
of the primary requirements from a military point 
of view is the ability to obtain a great deal of 
information in a short time. A detector with a fast 
response is, therefore, of greater use than one with 
a slow response, everything else being equal. 

As has been pointed out, not all the heat which 
falls on a detector is utilized in producing a re¬ 
sponse. This is borne out by the cases in which the 
ENI measured at long wavelengths is poorer than 
the measured ENI for 2-p radiation. It is, in gen¬ 
eral, true that the more uniformly black the detec¬ 
tor the more useful it is. It may be possible to 
increase the response if the blacking is improved. 

Because of the experimental difficulties attached 
to the ENI measurements, the zero-frequency re- 
sponsivity together with the frequency response 
might also be used as a basis for comparison. The 
responsivity in volts per watt was determined from 
curves such as are found in Figure 5, the gain of 
the amplifier, and the frequency response. The rms 
volts output is well above the noise and its value 
per watt of heat input peake-to-peak square wave 
may be read directly from the curve. When the 
volts per watt value, read from the curve, is divided 











COMPARISON OF THE DETECTORS 


by the amplifier gain and corrected for waveform 
and frequency fall-off, the responsivity in volts rms 
per watt rms into the amplifier is obtained. In com¬ 
paring the detectors in this way it must also be 
remembered that their usefulness depends on the 
noise inherent in them. If noiseless amplifiers of 
equal gain and noise pass band were used for all 
the detectors, the comparative noise output would 
be proportional to the square root of the ratio of 
the resistances. It might, therefore, be thought that 
for two detectors with the same zero-frequency 
responsivity used with noiseless amplifiers of equal 
gain and noise pass band, the one with the shorter 
time constant would be the more useful. This might 
not be true, however, because if the resistance of one 
were considerably larger than that of the other the 
noise output for it would be greater than for the 
other. It would, therefore, not be possible to detect 
such small signals before reaching the noise, or the 
ENI would be poorer. 

The above remarks are general and are introduced 
because it is difficult to compare on fair and equal 
bases a large number of devices of as great design 
diversity as those described in this chapter. Great 
caution must, therefore, be used with any table in 
which performance characteristics are compared. 
The information given in Table 23 is the result of 
measurements made under Contract OEMsr-1168 


277 


on certain detectors and probably cannot be used 
to generalize upon others of the same type. 

In view of the preceding remarks certain infor¬ 
mation on the best device of any particular group 
tested has been assembled in Table 23. Where only 
one unit of a type was on hand, as for example the 
Strong bolometer, the values for it are quoted. For 
information on any of the other examples, reference 
should be made to the appropriate section of this 
chapter. The detectors are listed in the order of 
increasing ENI at 15 c, using radiation predomi¬ 
nantly in the 5-p to 14-p region. Where two units 
have almost identical ENI, the one with the shorter 
time constant, or flatter frequency response, is rated 
higher. For each case listed in Table 23, the fre¬ 
quency at which the response is down 6 db (a factor 
of 2) from the response at 15 c is listed. Reference 
should be made to the frequency-response curve in 
the cases where no time constant is listed. Also 
listed are the spectral response, the responsivity in 
volts per watt at zero frequency, the resistance in 
ohms, the volts across the detector in operation, 
and the area in square centimeters. As the Golay 
device produces a voltage only when the heat cell is 
used in conjunction with a photocell and cathode- 
follower circuit, the responsivity listed for it must 
be thought of as the responsivity of the cell plus 
the photocell and the follower circuit. 


Table 23. Comparative values for various detectors. 



ENI 
(law at 
15 c) 

x (milli¬ 
seconds) 

Freqency 
at which 
respon¬ 
sivity is 
down 

6 db 
from 

15 c 

Spectral 
response in 
lp, to 14|i 

Respon¬ 

sivity 

volts/watt 

zero 

frequency 

Resist- 
tance 
in ohms 

Volts 

across 

detector 

Area in 
sq cm 

Cryostat No. 1 bolometer 

0.0006 

1.8 

145 

Uniform 

9.77 

0.3 

0.003 

0.00808 

Golay No. 2 heat detector 

0.0014 


72 

Uniform 

3.440.0* 



0.071 

BTL S-19 bolometer t 

0.004 


58 

Poor from 

2,700.0 

2.7x10® 

202.0 

0.00602 

Polaroid Ni324 bolometer 

0.01 


60 

3.5p to 6.5p, 

2.0 

64.0 

0.96 

0.045 

Felix No. 19 bolometer 

0.01 

19.0 

33 

Uniform 

2.0 

16.0 

1.7 

0.172 

Strong bolometer t 

0.03 


47 

Uniform 

0.3 

4.0 

0.96 

0.057 

RCA No. 18 bolometer 

0.02 

80.0 

30 

Uniform 


9.800.0 

1.2 

0.005 

Schwarz P4772/9 thermo¬ 
pile 

0.04 


40 

Cuts off 

0.72 

23.0 

0.0 

0.04 

Harris thermopile t 

0.20 


39 

at 10 
Uniform 

0.364 

100.0 

0.0 

0.11 

Weyrich thermopile 


66.0 

37 

Uniform 

0.29 

10 

0.0 

0.02 

Eppley thermopile 


90.0 

28 

Uniform 

0.375 

5.8 

0.0 

0.01 


* Responsivity measured at the output of the cathode-follower circuit shown in Figure 61. 
t Developed by Section 16.4. 










278 


FAR INFRARED DETECTING ELEMENTS 


811 RESUME 

The development of infrared detectors during the 
war period may be summarized briefly in the follow¬ 
ing manner. The development has been essentially 
in the field of thermopiles and bolometers. In the 
case of the former, the improvement is not phe¬ 
nomenal. Under Section 16.4, the Harris thermopile 
was developed which has very much the same re- 
sponsivity as thermocouples of the past but is some¬ 
what more rapid-acting. The Eppley thermopile 
is very nearly equivalent to the well-known Wey- 
rich unit used so extensively in spectrographic work. 
Both of the above are undoubtedly more rugged in 
character than the Weyrich type. The Schwarz 
thermopile developed in the Hilger laboratory may 
be regarded as a definite step forward in thermopile 
construction. The actual responsivity is roughly 
twice that of one of the former types. The Schwarz 
thermopile has the disadvantage of being easily 
broken. 

The greatest effort has been expended in the de¬ 
velopment of bolometers. Although well known, 
these were used only very seldom in the prewar era. 
Many variations of bolometers have been designed, 
but they fall essentially into two categories, the 
low-impedance and the high-impedance types. 

Examples of the low-impedance types are the 
Strong nickel-strip and the so-called Felix bolom¬ 
eter. The first of these was developed entirely under 
Section 16.4 and the second at least in part. Other 
low-impedance bolometers are the superconducting 


unit developed by Andrews and the Polaroid unit. 
The RCA instrument is of intermediate impedance. 
The BTL bolometer is a high-impedance unit devel¬ 
oped under Section 16.4. 

A third development is the Golay cell, which is an 
outgrowth of the Hayes cell. This device has been 
greatly improved in sensitivity and by the intro¬ 
duction of ballast cells was made nonmicro- 
phonic. 

The most sensitive of all the detectors developed 
is the superconducting bolometer worked out by 
Andrews which has an inherent noise level, so low 
that its ENI is only 6 X 10“ 4 pw when operated at 
15 c with a tuned amplifier of 2-c bandwidth. A 
rather close second is the Golay cell, for which a 
signal of 1.4 X 10~ 3 pw is equivalent to the noise 
of the instrument when operated under conditions 
equivalent to the foregoing. The other bolometers, 
when operated under similar conditions, are roughly 
equivalent to each other, all having equivalent 
noise inputs of about 10‘ 2 pw. A slight edge might 
be given to the BTL bolometers, of which one was 
found to have an ENI as low as 4 X 10" 3 pw. 

The superconducting bolometer and the Golay 
cell are about an order of magnitude better than 
any of the other detectors developed, the latter 
being only slightly better than prewar bolometers. 
The real advance has been in the improvement in 
frequency response to the point where a-c amplifiers 
might advantageously be employed. The use of a-c 
amplifiers virtually eliminates thermal drafts from 
the detecting system. 




Chapter 9 


FAR INFRARED RECEIVING SYSTEMS 
FOR MILITARY APPLICATIONS 

By Alvin H. Nielsen a and Harald H. Nielsen b 


91 INTRODUCTION 

Types of Military Application 

A ll bodies at temperatures above absolute zero 
emit electromagnetic radiations with wave¬ 
lengths in the infrared. Because most military in¬ 
stallations, industrial centers, ship convoys, etc., are 
hotter than their surroundings, detection of them 
by means of the self-emitted infrared radiations 
immediately suggests itself. The advantages of the 
method, namely that the detecting device does not 
betray its own position and that the heat energy 
emitted by military equipment lies principally in 
the wavelength interval or 8 to 14 p, which is not 
seriously attenuated by the atmosphere, suggest the 
importance of developing infrared devices for the 
following purposes: 

1. To detect personnel on foot or in small boats 
or landing craft. 

2. For mounting on the ground or in aircraft for 
the detection of tanks or other vehicles. 

3. For mounting on land to detect aircraft. 

4. For mounting in planes to detect industrial 
centers, factories, or concentrations of military ma¬ 
teriel in order to determine suitable targets for 
manual bombing or for attack with heat-homing 
bombs. 

5. For mounting on submarines or ships or at 
shore stations for detection and for determination 
of the bearing and the range of ships and other 
marine craft. 

6. To guide heat-homing missiles. 

Both the U. S. Army and Navy requested 
NDRC to set up projects with these developments 
as the goal. Under the auspices of Section 16.4, such 
projects were initiated at Western Electric Com¬ 
pany (BTL) under Contract OEMsr-636, at Har¬ 
vard University under Contract OEMsr-60, and at 

a Department of Physics, University of Tennessee, Knox¬ 
ville, Tennessee. 

b Mendenhall Laboratory of Physics, Ohio State Univer¬ 
sity, Columbus, Ohio. 


the University of Michigan under Contract NDCrc- 
185. Infrared receiving systems were developed 
under each contract for one or more of the purposes 
listed above and final reports on these developments 
have been submitted to NDRC. This chapter con¬ 
cerns itself with the discussion of the complete 
receiving units which, in all cases but one, employ 
detectors such as are described in Chapter 8 of this 
volume. 

The various devices which have been developed 
under the auspices of Section 16.4, NDRC, are sum¬ 
marized in the following paragraphs. 

The portable infrared detector [PND] was a de¬ 
velopment of BTL Contract OEMsr-636 for the use 
of military field personnel primarily to detect per¬ 
sonnel, vehicles, tanks, small boats, and ships. The 
instrument is a lightweight, self-contained, hand- 
pointed device which employs a two-strip ther¬ 
mistor bolometer situated at the focus of a parabolic 
mirror. A small vibrating mirror sweeps the target 
image back and forth from one bolometer strip to 
the other. The instrument has an instantaneous 
field of view about 70x10 minutes, and may be 
turned at about 2 degrees per second while search¬ 
ing for targets. The electrical system consists of a 
20-c signal amplifier and an 800-c bridge. The 20-c 
signal modulates the 800-c tone to give an aural 
signal in headphones or loudspeaker. Visual signals 
may also be had with a separate indicating unit. 
Section 9.4 contains a description of the PND and 
data on its performance. 

The scanning infrared detector [SND] was de¬ 
veloped by BTL under Contract OEMsr-636 for 
the purpose of locating moving tanks. It was 
designed to detect small temperature differences 
within a sector of 30 degrees scanned at the 
rate of 30 to 60 degrees per second. A two-strip 
thermistor bolometer is mounted at the focus of a 
scanning parabolic mirror, and the instantaneous 
field of view is from 0.5 to 4.0 degrees by 0.08 to 
0.16 degree, depending on the bolometer design 
used. The electrical system amplifies the signal 


279 





280 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


from the bolometer and presents it on waxed re¬ 
cording paper. Each signal causes a stylus to mark 
the paper, which advances a small distance for each 
scan. The target thus leaves a trail of marks on the 
record paper. A description of this equipment 
and results of the field tests are given in Sec¬ 
tion 9.5. 

The thermal map recorder for ground survey 
[TMR] is an airborne infrared receiver and re¬ 
corder developed by BTL under Contract OEMsr- 
636. It was designed primarily to permit night scan¬ 
ning and mapping of regions to determine if they 
contain suitable military targets for heat-homing 
bombs. The device, placed in the nose of a bomber- 
type aircraft, will plot a strip map of temperature 
differentials or discontinuities found on the earth 
over which the plane flies. It can be adjusted to 
provide a map with dimensions of equal length and 
breadth or one in which the length is some chosen 
factor of breadth. The forming of the map may be 
observed in flight. A description of the equipment 
and an account of some of the flight tests are given 
in Section 9.6. 

The thermal receiver with remote indicator (Type 
L), developed by BTL under Contract OEMsr-636, 
is a device for use in an airborne weapon. This 
weapon consists of a mother aircraft and a drone 
to be used for the destruction of enemy naval sur¬ 
face craft at night. By radar, primary search and 
location of the target are assumed to be accom¬ 
plished by the control aircraft preliminary to the 
release of the drone. After the drone is released it is 
controlled by radio. From its scanning device it de¬ 
rives positional information and data as to what 
course should be followed to cause a collision and 
the proper time for diving. This is presented on a 
suitable screen to the operator of the mother air¬ 
craft. Section 9.7 contains a description of this 
equipment and records of typical performances in 
the field. 

The far infrared bombsight with angular rate 
release [FIRBARR] is a device designed to scan 
and establish a line of sight to any heat target by 
using the infrared radiation from the target. The 
preliminary investigation of the practical possibili¬ 
ties of such equipment, carried out by BTL under 
Contract OEMsr-636, is discussed briefly in Sec¬ 
tion 9.8. 

The portable ship detector [PSD], which was the 
first of the receiving systems to be developed, is a 


small, manually pointed, scanning receiver for the 
detection and determination of the bearing of ships. 
It was developed by Harvard University under 
Contract OEMsr-60. The signals are detected by a 
thermistor bolometer mounted in the focal plane of 
a Schmidt optical system. After amplifying, they 
are presented as either an audible or a visible sig¬ 
nal. This equipment is described and information 
on its performance is given in Section 9.9. 

The stabilized ship detector [SSD] was developed 
by Harvard University under Contract OEMsr-60 
when it became clear that search was too difficult 
with the PSD mounted on the deck of a ship. It is 
a synchro-driven device utilizing thermistor bolom¬ 
eters in the focal plane of a Schmidt optical system. 
With this equipment it is possible to obtain good 
bearing accuracy and target resolution. A perma¬ 
nent record is made of the true bearings of all 
targets which are detected within the angle scanned. 
The SSD is described in Section 9.10 and represen¬ 
tative records of its performance are given. 

An intermediate infrared receiver [IIRR] was 
designed, assembled, and tested by the University 
of Michigan under Contract NDCrc-185 to investi¬ 
gate the practical usefulness of such equipment for 
the detection of military targets. It comprised an 
exploratory model of an intermediate infrared re¬ 
ceiving equipment utilizing a lead sulfide photo- 
conductive detector cell. Its design and perform¬ 
ance are discussed in Section 9.11. 

The far infrared rangefinder employing wave¬ 
lengths in the 8- to 14-p spectral region was de¬ 
veloped by Harvard University under Contract 
OEMsr-60. The equipment operates on the prin¬ 
ciple of the standard optical rangefinder, except 
that the “eyes” are metal-strip bolometers. With 
this device, ship ranges to 10 per cent or less, and 
bearings to ±1 minute of arc or less may be 
determined at night or in haze. It is effective to at 
least 5,000 yards. Automatic horizontal guiding was 
developed to make the apparatus useful on an 
invisible target. A description of the device and 
data on its performance may be found in Sec¬ 
tion 9.12. 

912 Aspects Common to All Systems 

All the foregoing pieces of apparatus have certain 
characteristics in common; for example, they are 
all equipped with more or less equivalent optical 




INTRODUCTION 


281 


systems, detectors, and amplifiers; they are sensi¬ 
tive to the same general interval of wavelengths in 
the spectrum; their efficacy of operation is essen¬ 
tially related to the thermal contrast between the 
target on which they are trained and the surround¬ 
ing background, etc. The following sections review 
briefly some of the aspects which the devices have 
in common and which are important in determining 
the excellence of their performance. 

Optical Systems, Detectors, and Amplifiers 

Each of the devices considered in this chapter is 
equipped with a light-gathering system which in 
turn concentrates the radiant energy on the detect¬ 
ing element. In the PND, the SND, the TMR, the 
Type L, the IIRR, and the infrared rangefinder, 
the light-gathering unit is a parabolic mirror, while 
in the PSD and the SSD a Schmidt assembly is 
used. In all cases the detecting element is located 
at the focus of the optical system. 

The choice of the optical system which is most 
suitable to a certain detection problem depends 
upon the “field of view” involved in the detection 
scheme. By field of view is meant the solid angle 
which is “seen” by the detector at a given instant 
during the scanning motion. Land-based equipment 
for scanning a sea horizon should have a very small 
vertical field in order to minimize noise and false 
signals originating in the background. On the other 
hand, homing devices require quite large fields of 
view. 

If a small field is satisfactory, the rays entering 
the optical system will be very nearly axial, and 
sufficiently good images will probably be formed by 
using a parabolic mirror to gather the light. If the 
field is large, however, the images which are formed 
off axis are important and a parabolic mirror may 
not be satisfactory. A spherical mirror with a re¬ 
fracting plate placed at its center of curvature to 
correct for spherical aberration forms what is known 
as a Schmidt system. Images formed by such a 
system are found to deteriorate slowly as the images 
go off axis. For fields up to 1 degree parabolic 
reflectors give sufficiently good images for most 
purposes. 

Except for the IIRR and the infrared range¬ 
finder, the detecting element is a thermistor bolom¬ 
eter developed by the BTL under Contract OEMsr- 
636. In the IIRR a lead sulfide cell developed at 
Northwestern University under Contract OEMsr- 


235 is employed, while the infrared rangefinder uses 
a nickel-strip bolometer of a squirrel-cage design. 
Every one of these devices has some method for 
vibrating the image of the target on and off the 
detector so that the signal produces a heating and 
a cooling of the bolometer, thereby effecting a 
pulsating current which facilitates amplification for 
recording or observation. Although the details of 
the amplifiers vary considerably from case to case, 
they have the same two essential parts, a pre¬ 
amplifier and a main amplifier. In all cases, except 
the IIRR, the detecting elements were made a part 
of a bridge arrangement which was included as a 
part of the preamplifier unit. In general, the main 
amplifier has been designed to have a narrow 
band-pass to reduce the inherent noise of the 
system. 

The current which flows through the bolometer is 
considered an average of the electron flow. Statis¬ 
tical departures from this average occur and with 
them occur fluctuations in the voltage across the 
bolometer, known as Johnson noise. This form of 
thermal agitation noise sets the lower limit on the 
signal which can be detected. The Johnson noise 
measured across a resistor will depend upon the 
temperature of the resistor, the magnitude of the 
resistance and the width of the frequency band in 
which the noise is measured. It is for this reason 
that the main amplifier is designed with narrow 
band-pass widths. 

The Importance of Atmospheric Attenuation 

The atmosphere contains certain amounts of 
water vapor, carbon dioxide and other CQmpounds 
of organic nature. These vapors all have character¬ 
istic absorption spectra. Because of the absorption 
bands of molecules in the air, the atmosphere is 
virtually opaque to infrared radiation at wave¬ 
lengths greater than 2.5 p except in a few “window” 
regions. Because objects of military interest have 
virtually all of their “incandescence” at wave¬ 
lengths greater than 3 p, only two of these windows 
are of interest, those from 3.0 to 4.0 p and from 8.0 
to 14.0 p. With the exception of the IIRR, all the 
devices described in Chapter 9 depend principally 
upon the radiant energy in the 8- to 14-p window 
for their operation. The IIRR operates on the radia¬ 
tion transmitted by the 3.0- to 4.0-p window. The 
attenuation of infrared radiation by the atmosphere 
is discussed in more detail in Section 9.2. 




282 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


The Importance of Contrast between Target 
and Background 

All of the receivers discussed in this chapter 
depend for their operation upon comparing the 
radiation received from a target with the radiation 
received from an equivalent amount of background. 
This comparison is affected by oscillating the target 
image off and on the detector strips. It is evident, 
therefore, that the contrast between the target and 
the background is of prime importance in the de¬ 
tection of objects at a distance by their self-emitted 
radiations. 

Drifting clouds along the horizon and light re¬ 
flected from the surface of the sea send out signals 
which tend to obscure a target. Variations in the 
background signals due to various meteorological 
or other causes are frequently referred to as back¬ 
ground noise if they are erratic or background 
signal if they are steady. (For further discussion, 
see Section 9.3.3.) 

92 ATMOSPHERIC ATTENUATION OF 
INFRARED RADIATION 

9,2-1 Introduction 

The transmission characteristics of the atmos¬ 
phere are of primary importance in determining the 
maximum range within which devices for signaling, 
for communicating, and for detecting objects of mili¬ 
tary interest can be successfully operated if they 
use self-emitted infrared radiation. It is, moreover, 
important to know how infrared radiation is trans¬ 
mitted by smokes of various kinds in order to deter¬ 
mine how effective a smoke screen might be against 
the detection of objects of military importance. 

The information previously available on the 
transmission of infrared radiation by the atmos¬ 
phere was only fragmentary, and the transmission 
through smokes was virtually unknown. To obtain 
more complete information on this subject for the 
Armed Forces, a project was requested in joint 
Army-Navy Project Control AN-32 and begun at 
Harvard University. 

It was shown that attenuation by water vapor, 
from the visible to 2.7 p, was due mainly to a deep¬ 
ening and widening of the water vapor bands, but 
that the attenuation between these bands was very 
weak. The measurements were carried out in condi¬ 


tions of high summer humidity and for very long 
optical paths, using the sun as a source. Measure¬ 
ments under varying degrees of haze were also 
made. The attenuation by the water vapor in a 
path length of 5,000 yards was small in the 8- to 
14-p region. Probably more important were the 
measurements made on this window region using 
the sun as a source just after it had risen from the 
sea. The transmission properties of several types of 
smokes were measured. 

9 2 2 A Resume of Earlier Work 

The attenuation of infrared radiation in the 
atmosphere is produced by the absorption of carbon 
dioxide and water vapor and by the scattering of 
fog, clouds, smoke, etc. For this reason only the 
wavelength between these absorption bands can be 
used for infrared transmission and the only impor¬ 
tant usable window is the previously mentioned 
8- to 14-p spectral region. 

The 8- to 14-p Window 

The 8- to 14-p window is bounded on the short 
wavelength side by the great water vapor vibration- 
rotation band centered at 6.2 p and on the long side 
by the carbon dioxide bands centered at 13.9, 15.0, 
and 16.2 p and also by the pure rotation spectrum 
of water vapor. There is an absorption band in the 
window at 9.5 p due to ozone in the upper atmos¬ 
phere, but this is of no importance for absorption 
paths along the earth’s surface. 

Radiation Absorption by Water Vapor. The 
8- to 14-p window is not perfectly transparent. The 
amount of attenuation is determined by the amount 
of water vapor in the path, even though no water 
vapor band is centered in this region. 1 The absorp¬ 
tion lines in a spectrum have a finite width and a 
shape resembling a resonance curve. With a very 
small amount of absorption, only the central peaks 
of the absorption lines are important; with a large 
amount, the wings of the absorption lines are also 
important and may produce a considerable absorp¬ 
tion far away from the line center. In the 8- to 14-p 
region there are no water vapor lines of any conse¬ 
quence, so that any water vapor absorption in this 
region must come from the wings of absorption lines 
lying at higher and lower wavelengths. The over¬ 
lapping of the wings of the many lines of the 6.26-p 
vibration-rotation band on one side of the window 





ATMOSPHERIC ATTENUATION OF INFRARED RADIATION 


283 


and the rotation lines on the other side serve to give 
a nearly constant absorption coefficient throughout 
the window region. Because of this lack of line 
structure in the window region, the absorption fol¬ 
lows Lambert’s law to a good approximation. 

If the transmission in the window region is less 
than 50 per cent, then Lambert’s law ceases to be a 
valid approximation and a more complicated ex¬ 
pression must be used. 1 A series expansion of this 
complicated expression is 

M = 0.45(0.53i) — 

E 4 

where A E/E is the fraction of absorbed radiation 
and the numerical constants have been chosen to fit 
experimental data. In this formula l is a measure of 
the amount of water vapor in centimeters of precipi- 
table water. If all of the water vapor in a column 
1 square centimeter in cross section along the ab¬ 
sorbing path were condensed into a cylinder of water 
of equal cross-sectional area, then the length of the 
water cylinder in centimeters is the number of centi¬ 
meters of precipitable water. 2 * 3 ’ 4 

Radiation Absorption by Organic Vapors. In 
addition to the absorption in the window region by 
water vapor there may be absorption by organic 
vapor also. 

It is known that many complicated organic mole¬ 
cules absorb heat radiations strongly and if the 
vapors which give the sea its characteristic odor, or 
associated organic vapors, also absorb strongly in 
the 8- to 14-p region, one might expect excessive 
attenuation in summer. These vapors are given off 
to the surface air more copiously by the warmer 
water, and they remain near the surface because the 
surface air is then more stable due to the even 
higher upper air temperatures. 

Radiation Scatteiing by Fogs. The transmission 
of far infrared radiation through fog has been 
found to be as poor as for visible radiation. 1 This 
seems strange when one considers the penetration 
of haze by near infrared radiation. A consideration 
of the particle size, however, will explain the situ¬ 
ation. Scattering by particles becomes important 
when the particle diameter is equal to, or larger 
than, the wavelength of the radiation under con¬ 
sideration. Haze particles are very small, being 
about 1 p in diameter. Such particles attenuate 
visible light but allow the near infrared radiation to 
be transmitted. Fog particles are about 20 p or 


larger in diameter and therefore are opaque to 10-p 
radiation in the window region. 

92 3 Recent Measurements on Atmospheric 
Attenuation of Infrared Radiation 5 

The attenuation of infrared radiation of various 
wavelengths from the visible to 2.7 p and through¬ 
out the 8- to 13-p window has been investigated 
with a recording prism spectrograph. 

The Radiation Sources 

As a source for the radiation of wavelengths less 
than 2.7 p, a tungsten filament lamp was used, and 
for greater wavelengths a heater was used as a 
source. The sources were mounted on a hill 50 feet 
above the sea at the focus of a 5-foot reflector at 
Fort Ruckman. The receiving equipment, including 
the spectrometer, was set up 5,000 yards away, 
across Broad Sound, at Fort Heath. In many of the 
experiments the sun was used as a source, the energy 
being concentrated on the spectrograph slit with the 
aid of a heliostat. The optical path in this case was 
determined by the zenith angle of the sun and was 
a maximum when the sun had just risen out of the 
sea. 

The Spectrometer 

The radiation was gathered by a collimating 
mirror and dispersed by a prism. For wavelengths 
less than 2.7 p a glass prism was used, and for 
longer ones, a prism of rock salt. The dispersed 
radiation was gathered by a telescope mirror which 
forms a spectrum at its focus. 

The telescope mirror and prism are rotated to¬ 
gether, causing the spectrum to sweep across the 
bolometer strips, which serve also in the capacity 
of an exit slit. A photograph of the spectrometer 
and telescope is shown in Figure 1. 

The Detecting and Recording System 

As a detector a bolometer was used made of two 
collinear strips % p thick and about % millimeter 
wide. The strips were about 6 millimeters long and 
were mounted in a cell with a NaCl window filled 
with hydrogen to a pressure of 5 mm of Hg. The 
strips were coated with aluminum black. A rotating 
disk was used in the collimating system to chop the 
incident radiation at a frequency of 10 c. 

The bolometer was connected electrically to a 






284 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


twin-T amplifier of about 0.5 c pass-band width. 
The a-c voltage of this amplifier is rectified by an 
electronic component which functions in such a 
manner that its d-c output voltage is very nearly pro- 



Figure 1 . Telescope and spectrometer for measure¬ 
ments of atmospheric attenuation of infrared radia¬ 
tion. 


portional to the logarithm of its a-c input voltage. 
This logarithmic output is fed to an Esterline-Angus 
recording milliammeter which records the action of 
the bolometer, or the hologram. Wavelength posi¬ 


tions on the holograms are determined by interpola¬ 
tion from the characteristic atmospheric bands. A 
typical hologram is shown in Figure 2. 

Method for Determining the Attenuation 

In determining the attenuation by a column of 
gas it has been standard practice among infrared 
spectroscopists to compare the energy transmitted 
through the gas path with that transmitted through 
an equivalent evacuated column. Such a procedure 
is, of course, impracticable here, since this would 
require an evacuated cell of 5,000 yards. An alter¬ 
native method is used and depends upon the follow¬ 
ing procedure. It is observed that the radiation at 
wavelength 2.16 p is independent of the amount of 
water vapor in the optical path and is, moreover, 
independent of the haze condition. This may be 
taken to mean that radiation of this wavelength 
comes through the atmosphere unattenuated under 
virtually all conditions. If now the form of the 
radiation curve of the source were known (it is 
probably not a black body) an envelope of the 
initial radiation could be determined using as a 
scale point the magnitude of the energy transmitted 
through the atmosphere at wavelength 2.16 p. The 
actual form of the radiation curve of the source is 
obtained by making a set of measurements on the 



- - 

KBIfl liR/ f ED 
















ATMOSPHERIC ATTENUATION OF INFRARED RADIATION 


285 


source at a distance of 14 yards, at which distance 
the attenuation on a dry day may be regarded as 
negligible. Such an envelope calculated for the 
incident radiation occurs as a continuous line above 
the attenuated curve in Figure 2. 

When the logarithmic response curves are con¬ 
verted to arithmetic the ratio of the areas under 
the attenuated response curves to the corresponding 
areas under the envelope curve serves as a measure 
of the radiation transmitted. 

While the 2.16-p region remains unattenuated 
also by haze this is not the case with the region 
near 0.86 p. This fact can be made to define the 
amount of haze present in terms of the difference 
between the intensity at 0.68 p, expressed in deci¬ 
bels, minus the intensity at 2.16 p. 



The hologram shown in Figure 3 has been con¬ 
verted from a logarithmic to an arithmetic response 
curve. In Table 1 are given the ratios, under vary¬ 
ing degrees of humidity and varying degrees of 
haze, of the areas of the attenuated response curves 
to the computed arithmetic response envelope 


curves which serve as a measure of the transmitted 
radiation. Bands I to V referred to in Table 1 are, 
respectively, at 0.70 to 0.92 p, 0.92 to 1.1 p, 1.1 to 
1.4 p, 1.4 to 1.9 p, and 1.9 to 2.7 p. 


Table 1 . Ratios of areas under arithmetic curves to 
area under envelope. 


Spectro¬ 

gram 

Water 

(cm) 

Haze 

(db) 

I 

II 

III 

IV 

V 

1 

1.1 

1.3 

0.91 

0.90 

0.79 

0.68 

0.57 

2 

3.5 

0.4 

0.82 

0.83 

0.67 

0.61 

0.55 

3 

7.5 

0.0 

0.75 

0.75 

0.59 

0.59 

0.54 

4 

7.3 

20.0 

0.79 

0.73 

0.58 

0.47 

0.54 


Figure 2 shows a solar hologram which indicates 
the transmitted spectral intensities to wavelengths 
of 12 p. The interval from the region marked x to 
the 4.3-p C0 2 band (Region VI) is of especial inter¬ 
est, since it marks a window in the near infrared. 
The region from 4.3 p to the great water vapor band 
(Region VII) delineates another window of less im¬ 
portance. The transmission by the atmosphere for 
wavelengths longer than 3 p may be obtained ap¬ 
proximately with the aid of the curve in Figure 2 
and the envelope which is constructed for a black 
body (the sun) at 6000 degrees K. 

The transmission between the regions marked Q 
and x is known to be high from earlier measure¬ 
ments. The apparent low transmission indicated for 
this region in Figure 2 is from falsification due to 
the slit width. This occurs when the slit is so wide 
that it subtends a sufficiently large wavelength in¬ 
terval so that the bolometer reading between Q and 
X is influenced by the absorption in these regions. 
In other words the absorption bands, and the 
window between, are not fully resolved. 

Similarly, unresolved ozone and water vapor 
bands account for the apparent low transmission in 
Regions I and II. 

Discussion of Results for Ship-Fired Smokes 

The final results for all ship-fired smokes are 
given in Table 2. 

These densities are all adequate to be completely 
opaque visually for the 1,000-watt searchlight. 

The FM, FS, and fog-oil smokes are less opaque, 
according to results obtained, than HC smokes, and 
by a factor of about 4. HC, FM, and FS have com¬ 
parable loss of opacity with increasing wavelength 
throughout the visible and near infrared. FS is rela- 




















286 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


tively more opaque in the far infrared than FM, 
but even so it is far from being an effective screen 
there; a plume opaque to our 1,000-watt searchlight 
attenuates only 4 clb at 9 p. 

Table 2. Results tor ship-fired smokes with relative 
ship-air velocity at 10 knots. 

HC smoke 


(1 pot = 32 lb of smoke or 32/6 lb per minute of smoke) 


X — 0.85 m- 

Attenuation per pot 

20 db 

2.10 m- 

4 db 

9.0 |x 

FM smoke 

1 db 

X = 0.85 p 

Attenuation for 30 lb FM/min 

18 db 

2.10 n 


9 db 

9.0 p 

FS smoke 

3 db 

X = 0.85 p 

Attenuation for 30 lb FS/'min 

15 db 

2.10 n 


4 db 

9.0 p 

Fog oil 

4 db 

X = 0.85 p 

Attenuation for 30 lb FO/min 

15 db 

2.10 p 

For a “parallel run” 

16 db 

9.10 p 

For a “parallel run” 

2 db 


Fog oil increases its transmission most rapidly 
with wavelength and at 2.2 or 9 p its attenuation is 
ordinarily negligible. In order to get any measur¬ 
able attenuation at these wavelengths, the smoke¬ 
generating ship was run almost parallel with the 
wind, quartering only enough to keep out of its own 
smoke. This produces a very dense plume of smoke. 
The generator put its maximum output in this 
plume for about 5 minutes. These fog-oil runs are 
referred to in Table 2 by the designation “parallel 
run.” 

The general conclusion that one comes to is that 
none of the smokes are effective for the far infra¬ 
red, or even for the intermediate infrared at ap¬ 
proximately 2 p. 

A study of smoke transmission has been made at 
the U. S. Naval Research Laboratory [NRL] and 
is covered by Report H-1602 of March 25, 1940. 
The results for HC smoke obtained at NRL and 
stated in this report agree generally with those 
obtained by the Harvard project. 

Test of Gaseous Screens for l = 9 p 

Field tests were carried out to determine if ap¬ 
propriate gases with absorption bands in the region 
8 to 14 p would afford effective screens. The three 
gases S0 2 , C 2 H 4 , and C 2 H G have strong absorption 


bands which together cover this spectral region. 
They were liberated from shipboard together with 
FM. The FM smoke was used as a marker to give 
evidence that the gases actually crossed the optical 
beam. 

Also NH 3 was liberated, in a second run, together 
with FM as a marker, to determine its efficacy as a 
far infrared screening gas. 

The regular 5-degree NaCl dispersing prism was 
used with the bolometer set at the maximum re¬ 
sponse, as previously used with the smoke screens. 

A combination of C 2 H 4 , C 2 H G , and S0 2 , liberated 
respectively at rates of 9, 9, and 4 pounds per 
minute, with FM marker, gave an immeasurably 
small attenuation although it was known that the 
gases actually crossed the optical beam. The result 
was similarly negative with NH 3 . The exact weight 
of the NH 3 cylinder could not be determined, since 
its weight (400 pounds gross weight) exceeded the 
capacity of the available scales. Its valve was 
opened wide for the run and a substantial amount 
of gas was discharged and crossed the optical path. 

93 THERMAL RADIATIONS FROM 
TARGETS AND BACKGROUNDS 

9,3,1 Introduction 

All natural and military objects may be said to 
be “incandescent” in the wavelength region from 8 
to 14 p. Because there is very little atmospheric 
attenuation of wavelengths from 8 to 14 p, radiation 
in the wavelength interval has been used for the 
detection of objects of military importance, for 
signaling purposes, and for guiding homing devices. 
The infrared detectors are, in general, made to com¬ 
pare the radiation received from a target with that 
from an equivalent area of the background, and 
their operation depends upon the differential in the 
emission from the two. 

If the background is uniform and unchanging 
the contrast between it and the target may be large 
and detection will be a simple matter. Usually, 
however, the background is not uniform, particu¬ 
larly over the sea. At the horizon will be found 
floating clouds which may be at temperatures dif¬ 
ferent from the sky. As they float by they emit 
signals. The waves themselves may reflect light to 
the detector which is interpreted as a signal. The 
water, which in general is at a temperature different 
from the target, may illuminate the target with 
radiation characteristic of its temperature. This 






THERMAL RADIATIONS FROM TARGETS AND BACKGROUNDS 


287 


footlighting may be seen by the detector and pro¬ 
duce a false signal. 

The background effects which have been named 
send out signals or noises which are detected by the 
receiver. Their net effect is to destroy the contrast 
between the background and the target, particu¬ 
larly when the target is distant. For the purpose of 
determining the ease with which military targets 
can be detected at various ranges by means of low 
temperature radiations, a survey of targets was 
undertaken at Harvard University under Project 
Controls AC-225.02, NO-183, and NS-163 (re¬ 
vised) . 

9 3 2 Emission of Targets and 

Backgrounds 2 

The detectability of a target is determined by 
consideration of its overall black-body temperature 
(or by the distribution of temperature over the 
surface of the target) together with the black-body 


means of a simple measuring device comprised of a 
thermopile at the focus of a mirror together with 
auxiliary electrical measuring equipment. Two 
auxiliary black bodies at known temperatures were 
used, for calibrations. 

Measurements of target temperatures were made 
with this equipment as follows: the thermopile was 
focused successively on the ship’s surface and on 
the two auxiliary black bodies. The latter were 
controlled so that their emission bracketed the 
ship’s emission. Three readings were obtained, cor¬ 
responding to the emission of the ship’s surface and 
the emission of the two black bodies. The equivalent 
black-body temperature of the ship’s surface was 
derived from these readings and the two known 
temperatures by linear interpolation. In order to 
cancel the effect of an air-path absorption, the 
auxiliary black bodies were placed, wherever pos¬ 
sible, at approximately the same distance as the 
tested surface. Some observations were taken with 
the measuring equipment near the ship’s measured 


8 

6 

V) 

Ld 4 
UJ H 
01 

O p 

S 

0 

-2 

4 

w 

UJ o 
UJ c 
01 

g o 

o 

-2 




■HULL 

MEASUREMENTS 


n 




STARBOARD HULL 

\ 



\ 





AIR 



TEMPERATURE 

^ 


■— o- 

-0-0 



PORT HULL 



AIR 



TEMPERATURE 



x 


~ -- - - 

- 0 —o 



STACK 


4:00 AM 
COLLIER: 

LENGTH 330* 
LAUNCHED- 1918 
BEAM 50' 

WEATHER: CLEAR 

SEA CALM 


Figure 4. Diagram showing measurements on tem¬ 
peratures of a target. 






z 

2 

X 

□ 

<ER 

GHTER 

:rs are fishing schooners 

>GES, TUGS (NOT STACKS)ETC 

\ 

\ 



0 FREI' 
OTHE 


\ 


DREC 


'V-L 






_ 







1 

\ 

\ 

v, 

^5 

< 



B 

_1_ 

□ 

1 




0 I_I_ __ 1 -- —L --- 

0 500 1000 1500 2000 2500 3000 3500 


YARDS 

Figure 5. Graphs of observed differential tempera¬ 
tures (ship minus background) against ranges. 


temperature of the target background. The tem¬ 
perature thus specified is the temperature of a black 
body which would have an emission equivalent to 
that of the target or background. 

Emissions and corresponding black-body tem¬ 
peratures have been determined for the various 
parts of a ship surface (and for backgrounds) by 


surface and some were taken from a station entirely 
separate and remote. In the latter case, the dis¬ 
tances, respectively, of the target and of the auxil¬ 
iary black bodies from the measuring apparatus 
were not equivalent. 

Figure 4 is an example of the results obtained on 
board or near ships in Boston Harbor and on board 


































































288 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


ships at sea. Stack temperatures ranged near 100 C, 
except an insulated stack which was about 20 C. A 
considerable variation in measured temperature, as 
determined from various viewing angles, is pro¬ 
duced by reflected sunlight and winds. 

The observed differential temperatures (ship 
minus background) are plotted in Figure 5 for var¬ 
ious ships as functions of their ranges. 

9,3,3 Background Noise and Background 
Signals 

The detection of ships by means of their infrared 
radiation depends on their thermal contrast against 
the background. When ships are at great ranges 
their emission may be confused with variations 
and fluctuations of the background radiation. For¬ 
tunately, as compared to land backgrounds, the sea 
provides a very uniform one. 

The word “variation’' is taken to mean the varia¬ 
tion of the apparent temperature of background 
from point to point at any given time. The word 
“fluctuation” is taken to mean change of the ap¬ 
parent temperature of the background with time at 
any given point. Variation might be produced by 
reflection in the sea of the radiation of the clear 
sky and fluctuations might be produced by reflec¬ 
tion in the sea at one point of drifting clouds. 

The variations and fluctuations of backgrounds 
may produce spurious response in receiving instru¬ 
ments. This spurious response is called background 
noise if it is erratic and background signal if it is 
steady. 

Origins of Background Noise and Background 
Signal 

Variations and fluctuations of the temperature 
immediately in front of an infrared detector may 
also give spurious signals and noise. The air, where 
it is transparent, does not emit infrared radiations, 
but it emits on either side of the 8- to 14-p window, 
where it is not transparent. Accordingly, inhomo¬ 
geneities of the temperature of air, near receivers, 
can produce spurious response. 

Such inhomogeneities have been found to produce 
fading in the signals obtained with the Type L de¬ 
tector. 6 Large signals faded to nothing within a 
%-second interval. The fading occurred in gusty, 
moist atmosphere but not in either alone. 


The study of background noise and signals is not 
a subject which can easily be treated in general, 
since the response, produced by variations and fluc¬ 
tuations of the background, of any particular re¬ 
ceiving equipment depends intimately on the field 
of view of the equipment and the method of scan¬ 
ning which it employs. 

Background variations and fluctuations can be 
most appropriately determined with the receiver 
which is to be used in practice or with a receiver 
which simulates the field of view and method of 
scanning to be employed. However, if fluctuations 
and variations are determined with a receiver which 
has a smaller field and a faster time constant than 
the field and time constant which characterize the 
receiver to be used in the field, then the fluctuations 
and variations which should be obtained with the 
field equipment can be predicted by integration. 
If, however, the field of view of the background 
measuring instrument is larger and if its time con¬ 
stant is slower, the results will not permit a com¬ 
plete prediction of the behavior of the actual re¬ 
ceiver to be used in the field. 



EFFECTIVE RADIATION TEMPERATURE 
IN DEGREES C 

Figure 6. Vertical variation of apparent sea and sky 
temperatures. 

Variations and Fluctuations of Background 

The ease with which a ship or target is detected 
at sea depends, in addition to the amount of radia¬ 
tion it emits, upon the radiation reflected from it. 
Consequently, a target is more readily distinguished 














THERMAL RADIATIONS FROM TARGETS AND BACKGROUNDS 


289 


under certain conditions of radiation than under 
others. These conditions will, of course, also affect 
the light reflected by the sky itself. 

Figure 6 shows the vertical variation of apparent 
sea and sky temperatures with clear and overcast 
sky conditions. These measurements were made 
from Castle Island in Boston Harbor and from a 
tall building in New York. They show the role 
which the reflected sky light plays to produce a 
vertical variation of apparent sea temperature near 
the horizon. From Figure 6 it is evident that a com¬ 
plete overcast greatly decreases this vertical varia¬ 
tion of background temperature. 



CLOUDLESS TO N AND E 
HAZY SKY 

HAZE LAYERS AT 3000 FT 


JU. 


V 

1 1 

_1_ 

j> 4000 FT - 

_1_1_ 



Figure 7. Background derivatives for Atlantic Ocean. 

The vertical variation of the apparent tempera¬ 
ture of the sea and sky background has been meas¬ 
ured with a compensated receiver arranged so that 
it is sensitive only to the gradient of background 
temperature. The receiver is shown diagrammati- 
cally in Figure 7, together with results obtained 
with it at elevations from 80 to 5,000 feet. The 


observations at 80 feet were taken from a coast 
observing station. The other observations were 
taken from the air. 

The contribution of reflected infrared radiation 
to produce fluctuations of the apparent sea temper¬ 
ature will depend on the reflectivity of sea water 
for the 8- to 14-p, radiations. Also, the contribution 
will depend on the character of the sea surface. 
This latter is locally dependent on wind and gen¬ 
erally dependent on wave motion. 

The qualitative application of the measured in¬ 
frared reflectivity to predict the effect of various 
sky conditions in producing variations and fluctua¬ 
tions of apparent sea temperatures is facilitated by 
the fact that this infrared reflectivity is very nearly 
the same as the reflectivity of water in the visible. 
With this fact in mind, one can employ the visible 
variation and fluctuation of the appearance of the 
sea to predict the effect of various sky and sea con¬ 
ditions in the infrared 8- to 14-p band. 

Hulbert 7 has recorded some of the characteristics 
of sea reflections. Particularly, he says, 

The light of the rim of the breezy sea, i.e., the sea from 
the horizon to 3 degrees below comes mainly from the region 
of the sky 25 degrees to 35 degrees above the horizon and 
hence the reflecting facets of the sea which are visible to 
the observer are tilted up on an average at about 15 degrees 
from the horizontal. ... A dark band of clouds rising up 
over the horizon does not darken the sea appreciably until 
it reaches an altitude of 25 degrees. 


9 3 4 Dependence of Target Emission 
on Meteorological Conditions 

The quantitative target-background temperature 
differentials reported above have been determined 
mostly in winter. Qualitative field observations 
made throughout the year indicate that the targets 
are relatively stronger in winter than in summer. 
For example, ships which are barely detected at 
a 5,000-yard range in summer have been detected 
at three times this range in winter. 

Although the water vapor content of a given op¬ 
tical path in Boston Harbor may vary by a factor 
of 7 or more from summer to winter, it is not ex¬ 
pected that the transmission for the 8- to 14-p band 
will vary enough to account for the summer-winter 
differences in range limits. It appears that other 
factors may be involved. 

Two possible factors for explaining the winter- 


























290 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


summer differences in range limits are: first, higher 
contrast may obtain between targets and back¬ 
grounds in winter (the cause of this is described 
later); and second, higher attenuation due to higher 
concentrations of organic vapors characteristic of 
sea air may obtain in summer. 

Footlight Effect 

The vertical surface of a ship on a flat sea is ex¬ 
posed to radiation transfer from the warmer water 
in winter throughout a solid angle approximating 
jt steradians; whereas, the detector is not exposed 
to such transfer because it “sees” the water at graz¬ 
ing incidence. It is because the water illuminates the 
ship’s surface and cannot be seen that this effect is 
called the footlight effect. 

For example, sea and air temperatures for winter 
and summer off Yokohama 8 are: 



Summer 

Winter 

Sea 

25 C 

17 C 

Air 

25 C 

3 C 

Difference 

0 C 

14 C 


In summer and winter the cooling by so-called 
nocturnal radiation there is about the same. Thus, 
in summer, a vertical ship surface initially at the 
air temperature is in radiative equilibrium with 
the sea and is neither heated to a higher tempera¬ 
ture nor cooled to a lower temperature by it. 
Whereas, in wdnter, a vertical ship surface ini¬ 
tially at the air temperature is heated by the radia¬ 
tion from the sea to a higher-than-air temperature. 
For still air and for a 14-degree sea-air differential 
this effect is calculated to heat the vertical surface 
about 2 degrees above air temperature (or 2 de¬ 
grees above the temperature it would otherwise 
have). Moreover, the heating of crew’s quarters in 
winter adds to making a ship a relatively warmer 
target then. 

Considered together with the greater water vapor 
and possible greater organic vapor attenuation in 
summer, these effects all operate to make winter 
signals produced by ship targets (viewed along a 
horizontal optical path) relatively much stronger 
than summer signals. When these factors have been 
quantitatively evaluated, especially since the new 
measurements of transmission reported in Section 
9.2 are available, the detectability of ships under 
all meteorological conditions should be predictable. 


9 * PORTABLE INFRARED DETECTOR 
Introduction 

The portable infrared detector [PND] 9 was de¬ 
veloped by BTL under Contract OEMsr-636 as 
Project Controls N-108 and CE-37 for use by 
military field personnel for the detection primarily 
of personnel, vehicles, tanks, small boats, and ships 
by means of the intrinsic difference in temperature 
between the object and its immediate background. 
This development was initiated following sugges¬ 
tions by officers of the Engineer Board, Bureau of 
Ships, and Bureau of Ordnance that a model of a 
portable far infrared receiver be constructed for the 
purpose of testing whether a man could be detected 
at night through the reception of thermal radiation 
from his body at ranges of 100 to 200 yards. 

During the course of the development the scope 
of this project was enlarged from the original pur¬ 
pose by later requests from the Armed Services to 
include the investigation of the most advantageous 
utilization of thermistor bolometers for detection 
of infrared signals, the development of associated 
receiving equipment, including electronic circuits 
and optics, and the application of such equipment 
for Armed Services needs, particularly the detection 
of the types of military targets referred to above. 

Due care was taken in this development to assure 
a lightweight, compact unit, but no attempt was 
made to design the equipment for extremes of oper¬ 
ating conditions such as very high and very low 
ambient temperatures, total immersion in water, 
extremely rough handling, etc. 

This development has also served as a basis for 
the construction of the following related types of 
equipment under Contract OEMsr-636: the scan¬ 
ning infrared detector (see Section 9.5), the ther¬ 
mal map recorder for ground survey (see Section 
9.6), and the thermal receiver with remote indi¬ 
cator (see Section 9.7), as well as for the prelimi¬ 
nary survey on the far infrared bombsight with 
angular rate release (see Section 9.8). 

General Description 

The PND consists of two units, as shown in Fig¬ 
ure 8, and may be carried and operated by one 
man. The first unit, which weighs only about 10 
pounds (exclusive of tripod), contains the optical 











PORTABLE INFRARED DETECTOR 


291 


system, double-strip thermistor bolometer, ampli¬ 
fier, headphone signal indicator, and the batteries 
in a cylindrical case. This unit is a convenient 
device for the detection of persons or other warm 
targets by giving aural signals. The second unit, 
which also weighs about 10 pounds, contains an 
auxiliary amplifier, loudspeaker, and visual indi¬ 
cator for use in lieu of headphones. 



Figure 8. Photograph of portable infrared detector. 

In operation the field of view is focused upon the 
parallel thermistor strips by means of an 8-inch 
parabolic mirror of 6-inch focal length and a small 
modulating plane mirror. The latter is vibrated so 
that it will focus the images of the target and of 
the adjacent background alternately upon each of 
the two bolometer strips 20 times a second in order 
to produce an a-c signal and to make the signal 
independent of any difference between the local 
temperature at the PND and the background tem¬ 
perature of the object viewed. The resulting a-c elec¬ 
trical signal from the bolometer strips is amplified 
by a 20-cycle amplifier and made to unbalance an 
800-cycle bridge. The unbalance signal from this 
bridge may be heard with headphones or further 
amplified in the second unit for the operation of an 
indicator light or loudspeaker. 

The field of view of the bolometer is 70 minutes 
in one direction and 6 to 10 minutes in the other 
direction. The minimum detectable radiant power 
incident on the reflector is about 1 X 10~ 7 watt. 
With this system, accurate results can be obtained 
by manually scanning over a plane angle of 2 de¬ 
grees per second. Informatory but less accurate 
results can be obtained at a rate of 20 degrees per 
second with the plane mirror stationary. 


913 Description of Component Parts 
of the PND 

Optical System 

Reflectors. The optical system has been described 
in a previous paragraph. 

Bolometer and Angle of View. The bolometer 
consists of two thermistor strips, each 3x0.2 milli¬ 
meter in size and mounted parallel to each other, 
behind a protecting window of NaCl with a spacing 
of 0.6 millimeter center to center, on a backing of 
glass or quartz. The field of view would be, for 
perfect optics, 0.002 radian along the strip and 
0.0013 radian across the strip with an angular sep¬ 
aration of 0.004 radian between the strips. Because 
the optics are not perfect the effective widths of the 
fields of view for each strip are somewhat larger, 
the circle of confusion of the mirror being about 0.5 
millimeter. 

The vibrating mirror is in its extreme positions 
most of its time. When the mirror is at the one ex¬ 
treme, the projection of one of the thermal strips, 
A, will be at location 1 and the projection of the 
other strip, B, will be at location 2. When the mir¬ 
ror reaches the other extreme position, the projec¬ 
tion of thermal strip A is shifted to location 2 and 
the projection of thermal strip B is shifted to loca¬ 
tion 3, locations 1, 2, and 3 being closely adjacent 
to each other in order. If the integrated radiation 
is the same from each of the positions 1, 2, and 3, 
then no signal will result. Any difference such as 
that due to the presence of an object in the pro¬ 
jected image of the strip at either position 1, 2, 
or 3 will give a signal and the strongest sig¬ 
nal will be obtained when the object is in posi¬ 
tion 2. 

Vibrating Mechanism 

The ideal vibrating motion for the mirror is a 
periodic square wave of fixed amplitude and fixed 
period. This motion was accomplished by means of 
an electromagnetically driven torsional oscillator 
consisting of a piano wire, 4x0.16 inch, loaded by a 
soft iron armature to give the desired natural fre¬ 
quency. 

A steel bar fastened to the wire is limited in its 
motion by a pair of stops at each extremity, and 
the small mirror, which is mounted on this bar, is 
therefore also limited in its amplitude of motion. 





292 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


The steel bar spends most of its time at the stops 
and is quickly switched between the stops so as 
to produce a nearly square wave of motion at 
20 c. 

Detecting Element and Amplifier 

Detecting Element. The closely matched thermis¬ 
tor strips which constitute the detecting element 
have a resistance between 1.5 and 3 megohms. The 
electrical characteristics of this type of bolometer 
have been discussed in Section 8.7. 

Amplifier. The amplifier and batteries are con¬ 
tained in the cylindrical case just behind the main 
parabolic mirror, and the first stage of the ampli¬ 
fier is centered within this cylinder and shielded by 
a Permalloy case. The total amplification is pro¬ 
duced in three stages, the resistance-capacitance 
coupling being so proportioned as to yield a fre¬ 
quency characteristic selective with respect to 20 c. 
The maximum 20-c gain is somewhat greater than 
85 db, with the gain falling off by 6 db at frequen¬ 
cies of approximately 10 and 50 c. A suitable gain 
control is provided. 

Following the three-stage selective amplifier is a 
bridge configuration employing the plate impedance 
of one of the miniature tubes as one of the arms. An 
800-c oscillator is employed to excite the bridge, 
which is brought to a balance when no signal falls 
on the detecting elements. When a heat source is en¬ 
countered, the vibrating mirror shifts the image 
back and forth across the bolometer strips at the 
rate of 20 c as already described. The application 
of this signal unbalances the bridge with the result 
than an 800-c tone, modulated at 20 c, is heard in 
the listening device. 

Overall sensitivity of the amplifier is such that 
thermal noise due to the approximately 2-megohm 
input resistance presented by the bolometer pro¬ 
vides audible modulation of the 800-c tone. Detec¬ 
tion of a signal input of somewhat less than 1 
microvolt is readily possible with the headphones, 
the distinctive 20-c modulation due to a signal 
being distinguishable even when several decibels 
below the average thermal noise level. 

The battery supply arrangement for the bolom¬ 
eter unit is such as to bring the common connection 
of the two strips to the amplifier to approximately 
zero d-c potential with respect to ground, a condi¬ 
tion which aids in minimizing microphonic effects 
resulting from movement of this lead. 


Auxiliary Indicator Unit 

Where the wearing of headphones is undesirable, 
the auxiliary indicator unit provides a loud-speaker, 
together with the additional gain required to attain 
about the same threshold sensitivity (under ordi¬ 
nary noise conditions) as that obtainable with the 
phones. This unit is compact and relatively light 
(about 10 pounds) and provides, in addition, a 
visual indicator which is particularly useful in noisy 
locations. 

For aural indications, a two-stage amplifier 
serves to energize a small permanent-magnet type 
dynamic speaker. The frequency characteristic of 
the amplifier is such as to discriminate against both 
20 cycles and harmonics of 800 cycles. For visual 
indications, a rectifier is employed to actuate a sen¬ 
sitive relay according to the envelope of the modu¬ 
lated 800-c signal applied to the amplifier. The 
relay, in turn, closes the circuit of a type 313A gas 
tube which glows with an orange color. The tip of 
this tube is visible at the top of the unit. As with 
aural indications, visual detection of signals below 
the existing noise threshold is possible, the rhyth¬ 
mic flickering of the glow tube on a true signal 
being distinguishable from the random flickering 
occasioned by noise peaks. 

Operation and Operational 
Limitations 

Operation 

When a region is surveyed, the apparatus accepts 
all the radiation from the region that is not ab¬ 
sorbed by the atmosphere, lost on reflection, or ab¬ 
sorbed by the NaCl window on the bolometer. This 
includes actual radiation coming from the region 
viewed due to its absolute temperature and emis- 
sivity plus any reflected or scattered radiation fall¬ 
ing on the viewed region from other sources. The 
apparatus compares this total radiation from a 
given object with the total radiation from each side 
of the object. Whether or not a signal is obtained 
depends on the magnitude of this difference. The 
response of the apparatus is not a linear function 
of signal strength. The response increases at first 
rapidly with increase in radiation strength and then 
saturates. The gain provided when the gain ad¬ 
justment is maximum is sufficient to bring out the 
resistance or Johnson noise, tube noise, etc. Most 







PORTABLE INFRARED DETECTOR 


293 


regions when viewed will give a multitude of signals 
when the gain is wide open. Exceptions to this are 
a clear sky, open water, or other uniform back¬ 
grounds. On a bright day objects of all kinds give 
signals. On an overcast or rainy day and at night, 
contrasts due to reflected radiation are at a mini¬ 
mum. It therefore follows that proper operation 
under such varying conditions depends on proper 
adjustment of the gain to a point where signals are 
not so numerous as to defy interpretation. 

In use one can orient the apparatus so that the 
long dimension of the strips is at any angle that 
will give additional advantage in detecting a par¬ 
ticular object. For example, if the background con¬ 
sists of objects that have a large vertical dimension, 
such as tree trunks, then each one gives a signal 
when the strips are vertically oriented, and if one 
is looking for a man or house back in the trees, this 
is difficult. If, however, the apparatus is oriented 
so that the strips are horizontal, then the tree 
trunks give no signal, but the man or house will, 
unless completely obscured. With the length of the 
strips in the horizontal direction, the horizon will 
give a signal. For a single observer in a locale of 
low ambient noise, the earphones are probably the 
best. In places of high ambient noise level, the vis¬ 
ual signal obtained by using the indicator unit is 
by far the best. The aural signal from the loud¬ 
speaker is useful only in low ambient noise when 
several observers are present. 

The small angular field of view of the PND 3 is 
very useful in locating the bearing of signals accu¬ 
rately. On the other hand, this field of view makes 
it easy to miss a target and also hard to hold one 
when found if operating on a nonstable platform. 

While the apparatus is designed to work with the 
vibrating mirror, it has been found that it can also 
be operated with the vibrating mirror turned off. 
In this case, a signal is obtained when the image 
of the target is made to cross the bolometer strips. 
The optimum rate of motion is determined by the 
nature of the signal voltage transient and the pass 
band of the amplifier. This rate turns out to be 
about 20 degrees per second, as compared with 2 
degrees per second when the vibrating mirror is on. 
In this way one can get information much faster. 
This method is especially useful where two or more 
prominent targets are moving with respect to each 
other, and one wishes to keep track of their approx¬ 
imate relative positions. This scanning method has 


also been used with an experimental recorder at¬ 
tached to the apparatus. It is found that there is a 
time delay between crossing the target and the ar¬ 
rival of the signal of about 0.05 second. Without 
the recorder, accurate bearing cannot be obtained 
with this scanning method. 

Operational Limitations 

Ambient Temperature. The lowest ambient tem¬ 
perature at which the equipment will operate satis¬ 
factorily was determined by the lowest temperature 
at which the batteries would operate. Because of 
certain materials used in construction, the PND 3 
should not have been subjected to temperatures 
above 120 F. 

Weather. Because the system is not strictly water¬ 
tight, long exposure to high humidity would prob¬ 
ably be detrimental. The rock salt window has a 
protective coating that will take actual drops of 
water on it without harm; however, the continuous 
presence of a water film on it would probably be 
destructive. The presence of a film of water cover¬ 
ing any of the optical surfaces absorbs most of the 
useful radiation and greatly cuts the sensitivity. 
Experience has, however, shown that with a bright 
outside finish the apparatus will rarely get below 
the dew point. The apparatus will even operate in 
light rain unless the rain is blowing directly into 
the opening. As to dirt or tarnish on the optical 
surfaces, it is surprising how bad this will appear 
to be without appreciably affecting operation. It is 
strongly recommended that no attempt be made to 
clean any of these surfaces. If they become covered 
with a film of water, it is best to allow these films 
to dry by placing the apparatus temporarily in a 
dry place. Because of the construction the appara¬ 
tus will probably not function very well in a 
head-on wind or air blast of any great velocity. 

Shock. As regards extreme shock, such as drop¬ 
ping the apparatus, the controlling limitation will 
be how much damage will be done to the precision 
of the parabolic reflector. 

Viewing through Windows. Use of the apparatus 
from the inside looking through almost any type of 
window will not prove satisfactory, since windows 
of glass and almost all thick materials will absorb 
most of the useful radiation. 

Hot Sources. Moreover, the bolometer is a sen¬ 
sitive instrument and it has not been made to meas¬ 
ure very hot sources. The local temperatures that 








294 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


will result from pointing it directly at the sun will 
permanently ruin the bolometer. 

Background Compensation. The PND was aimed 
at both very cold and very hot backgrounds, and 
the output noise was compared with the noise ob¬ 
tained when the radiation opening was covered. 
The compensation was not perfect, but for reason¬ 
ably cold or hot backgrounds the increase in noise 
was less than a factor of 2. The greater the dif¬ 
ference between background and ambient tempera¬ 
ture, the greater would be this additional noise. 

9 45 Range and Sensitivity 

The range and sensitivity would naturally depend 
upon the optical alignment of the system. The align¬ 
ment procedure was simple, however, and, when 
once adjusted, only a severe shock would knock it 
out of adjustment. 

Minimum Signal 

By centering the PND on a small black-body 
source and reducing the source temperature, it was 
found that a signal of 0.1 or 0.15 pw could just be 
detected aurally. This corresponded to a flux den¬ 
sity of 400 to 600 ppw per square centimeter at the 
parabolic reflector. 

Range 

The maximum range of operation of the PND de¬ 
pends upon the size of the target, its contrast rela¬ 
tive to its background, and the atmospheric con¬ 
ditions. For example, hot objects, such as power 
plants, hot chimneys, etc., which are characteristic 
of regions of high population density, can be de¬ 
tected through haze that completely obscures vision. 

Field tests carried out in cooperation with the 
Army Engineer Board in December 1943 and in 
March 1944 at Fort Belvoir showed the following 
typical ranges for the detection of personnel at 
night under reasonably good weather conditions: a 
man’s hand at 500 feet; a man at 1,000 to 2,000 
feet; three men clearly resolved at a range of 1,500 
feet, even though the end men subtended an angle 
of only three degrees, and two men in a rubber 
boat at % mile. In tests made by Engineer Board 
personnel during July 1944, in the jungle near Fort 
Pierce, Florida, a man was detected at a range of 
50 yards while so well hidden in tropical foliage 
that he could not be seen visually or detected in 
photographs. 

In tests carried out at Lakehurst in cooperation 


with the Bureau of Aeronautics, the PND, mounted 
in a blimp, despite severe acoustical noise and vi¬ 
bration, definitely detected a coastwise vessel at a 
range of three miles. 

In March 1944, tests of the PND mounted in 
B-26 aircraft were made at Floyd Bennett Field 
in cooperation with the Bureau of Ordnance. At 
4 P.M. at an altitude of 10,000 feet, with light haze 
and smoke present and the ground partly covered 
with snow, no signals were obtained from the water, 
but distinct signals were obtained from a large gen¬ 
erating station, clusters of houses, wide roadways, 
and ships in the East River. In a field test at Dam 
Neck, Virginia, carried out in cooperation with the 
same bureau, to determine the possible usefulness 
of the PND as an antiaircraft gun detector at night, 
the PND detected a TBF plane flying at an altitude 
of 2,000 feet at a range of about 4,000 yards. 

In field tests at the Bureau of Ships Test Sta¬ 
tion, Cape Henlopen, Delaware, in February 1944, 
of the land-based PND under favorable atmospheric 
conditions, the minimum range of approach without 
detection of an LCI (bow or stern view) was about 
8,000 yards and of an LCT (bow or stern) was 
about 7,000 yards. With more favorable ship as¬ 
pects, the LCI was reliably detected up to a maxi¬ 
mum range of 11,000 yards and the LCT to 8,500 
yards. 

More intense heat sources, such as power plants 
and larger ships, should be capable of detection at 
considerably greater ranges. For example, in the 
Cape Henlopen tests, carried out with the Bureau 
of Ships, a cargo vessel was reliably detected to a 
maximum range of 17,000 yards. 

9 4,6 Present Status 

As a result of the field tests carried out at various 
places and under greatly varying conditions, the 
Army Engineer Board has placed a commercial 
order for pilot production of Penrod, which was the 
Engineer Board model of the PND. One-half of the 
initial order is allocated to the Bureau of Ships. 

95 SCANNING INFRARED DETECTOR 
[SND] 

Introduction 

The imperative need of the Army for a reliable 
airborne scanning and recording equipment pri¬ 
marily for the detection of tanks, and the relative 
security with which an infrared receiver may be 





SCANNING INFRARED DETECTOR [SND] 


295 


operated, led to preliminary tests with existing in¬ 
frared receivers at Fort Belvoir in September 1944. 
The equipment used consisted of a Model 3 PND 
(see Section 9.4) with headphones, a Model 3 PND 
operating with a waxed-paper recorder, and a Type 
L scanner-detector system (see Section 9.7) with 
cathode-ray presentation. 

The promising results of these preliminary tests, 
together with the prior observation that useful sig¬ 
nals could be obtained with the PND unit with 
the modulating mirror at rest if the line of sight 
of the device were waved (at an optimum scanning 
rate of about 20 degrees per second) across a heat 
source, led to the development by BTL under Con¬ 
tract OEMsr-636 of the scanning mfrared detec¬ 
tor [SND] 10 as Project Controls CE-37 and AC- 
225.020. The development was to provide an air¬ 
borne, scanning infrared receiver with a suitable 
form of recorder in order to permit tests of such 
equipment for the detection of tanks, ships, and 
such othei; military targets as deemed desirable by 
the Armed Services and the NDRC. 

Field tests (as described in Section 9.5.4) of this 
equipment have been carried out at night with the 
SND land-based, for the detection of tanks, hillside 
embrasures, and ships; with the SND airborne for 
the detection of tanks at night and ships in day¬ 
light, and for the mapping of land and shore-line 
terrain both at night and in daylight. 

General Description 


wax-paper chart to record the azimuthal locations 
of the heat sources from which signals are received. 
The position of the recorder stylus is mechanically 
synchronized with the angular position of the scan- 


A. SCANNING HEAO AND RECORDER 



The exploratory research model SND constructed 
and set up for operation, as shown in Figure 9, 
consists of four sections (exclusive of the standard 
surveyor’s tripod to hold the scanner-recorder). 
The parts are interconnected by means of shielded 
cables and are identified as scanner head and 
recorder, weight 22 lb; amplifier and control box, 
weight 20 lb; power converter and cable box, weight 
23 lb; and 24-v storage battery box, weight 24 lb. 

To meet various field conditions the equipment 
was made flexible as to type of bolometer used, 
scanning speed, electric passband, etc. 

In operation the field of view is focused upon the 
thermistor bolometer detector by means of a para¬ 
bolic mirror. The optical system and detector con¬ 
tinuously scan a sector 30 degrees wide at a rate of 
30 to 60 degrees per second. The amplified bolom¬ 
eter output actuates an electromagnetically oper¬ 
ated stylus which makes a dot or short dash on a 


Figure 9. Photograph of scanning infrared detector. 

ning head. The wax-paper chart is advanced at the 
end of each scan so that a time record of the re¬ 
ceived signals is obtained. The record reproduced in 
Figure 11 shows, for example, the type of record 
obtained from two moving tanks. 

A set of seven plug-in units, having “center fre¬ 
quencies” in the range from 27 to 150 c, controls 
the frequency passband and permits the selection 
of the optimum passband for the type of scene 
scanned. 

For the set of instrumental conditions which give 
maximum sensitivity, the electric passband is cen¬ 
tered around 27 c, the bolometer has a time con¬ 
stant of 7 milliseconds, and the scanning speed is 
30 degrees per second. With this arrangement the 
minimum detectable radiant power incident on the 
7-inch diameter reflector is 8.1 X 10“ 8 watt. 














296 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


9 5 3 Description of Component Parts 
of the SND 

Optical System 

In the SND the bolometer unit is mounted rig¬ 
idly at the focus of a gold-surfaced, glass parabolic 
mirror. The mirror is 7 inches in diameter and has 
a focal length of 6 inches and a circle of confusion 
of 0.25 millimeter. The mirror size and focal length 
were chosen after consideration of the available 
bolometer flake sizes, target size, circle of confu¬ 
sion, scanning speed, etc. The field of view of the 



Figure 10. Photograph of scanning infrared detector 
(front view). 


optical system may be varied from 0.5 to 4 degrees 
high and from 0.08 to 0.2 degree wide, depending 
upon the bolometer flake size in the various plug-in 
bolometer units. For point sources on average ter¬ 
rain the desirable field is about 1.0x0.08 degree. The 
optical assembly is mounted within a cylindrical 
aluminum case on top of which is mounted the pre¬ 
amplifier. 

Figure 10 is a photograph showing the front of 
the scanning head assembly when mounted on a 
tripod. The principal elements of the assembly are 
titled in this photograph. 

The mirror and bolometer housing takes the 
rather simple but sturdy form of 2 thick circular 
ribs bolted together to the interior of an aluminum 
alloy tube. Adjustment of the optics is accom¬ 
plished by moving the mirror back and forth or 
tilting it with the adjustment screws. 


Scanning Mechanism 

Scanning is accomplished by oscillating the whole 
optical assembly with a motor-driven cam mecha¬ 
nism over a 30-degree sector at constant speed. 
Having mounted the optical parts and having pro¬ 
vided an electric path to use the signal from the 
bolometer, it was then necessary to rotate the scan¬ 
ning head through an arc in a cyclic fashion in 
order to provide the heat transient which would 
upset the electric balance of the bolometer bridge. 
Mechanically, this scanning process could be per¬ 
formed most simply with a mechanism which pro¬ 
vided a sine wave displacement function (in de¬ 
grees). On the other hand, from the standpoint of 
the results desired, it would be preferable to have 
the scanning velocity, either right or left, a con¬ 
stant with time. The displacement function for this 
type of scanning if plotted versus time would then 
look like a sawtooth. When using the sine wave 
function for scanning, only perhaps 50 to 70 per 
cent of the total time would involve & scanning 
rate which was reasonably constant. On the other 
hand, the use of a sawtooth function for scanning 
would involve rather large accelerations at each 
turn-around point and all sensitive elements 
mounted on the scanning head would be subjected 
to the shock of vibrations set up at the turn¬ 
around point. The type of scanning function which 
has been decided upon in the SND is a compromise 
between the two types which have been described. 
The turning velocity of the head in degrees is rea¬ 
sonably linear over about 30 degrees but 2% de¬ 
grees are added for each turn-around point in the 
cam-drive design. 

The scanning speed was from 30 degrees to 60 de¬ 
grees per second, being dictated chiefly by the re¬ 
sponse time and size of available bolometers; the 
recording paper advanced % 5 inch at the end scan. 

Detecting Element and Amplifiers 

Detecting Element. Several different dispositions 
of paired bolometer flakes were used in a plug-in 
type assembly. For maximum sensitivity to small¬ 
sized sources on average terrain, two vertical strips, 
one above the other, were used. Each strip was 
from 1 to 5 millimeters high and 0.2 millimeter 
wide, the active areas overlapping slightly. In one 
of the dispositions the bolometers were obliquely 
mounted so as to permit an overlap in order that 









SCANNING INFRARED DETECTOR [SND] 


297 


there would be no dead spot at the center. The 
strips were aligned vertically so that an extended 
vertical source, such as a tree, would cross both 
strips at approximately the same time and give no 
signal through the bridge circuit. Small sources 
which cross only one strip would give a full strength 
signal. In another disposition the bolometer had 
only one active strip and gave a truer heat picture. 
Another possibility is to construct the bolometer 
with two active strips mounted side by side. A dis¬ 
cussion of the characteristics of these bolometers 
may be found in Section 8.7. 

In the case of the SND it was decided that the 
abrupt changes in the heat picture produced by 
boundaries and by small hot or cold sources would 
be the most important types of signals to record. 
The design was, therefore, based on the intention 
of making detection ability of the SND optimum 
for localized heat sources which were small enough 
to be considered point sources. On this basis, it 
turned out that the optimum results would be ob¬ 
tained when the bolometer time constant was ap¬ 
proximately equal to the exposure time. With the 
SND scanning speed of 60 degrees per second, a 
3-millisecond time constant was approximately 
right. The bolometer, which has a resistance of 
about 2.5 megohms is made part of the bridge net¬ 
work. 

Amplifiers. The SND contains a preamplifier and 
a main amplifier and rectifier. The bolometer bridge 
and the preamplifier to which it is connected are 
housed in the scanning head. The preamplifier has 
a voltage gain of 50 db at 100 c from a bolometer 
bridge source having arms of 2-megohm resistance 
each. The signal is then fed by means of a cable 
to the main amplifier, which is in a separate unit 
and which has a maximum voltage gain of 100 db 
at 100 c. 

This RC-coupled main amplifier has a built-in 
pulse-shaping network to take into account ampli¬ 
tude distortion, frequency response, and phase dis¬ 
tortion. In order that the pulse-shaping action 
should be determined by the pulse-shaping plugs, 
it was necessary that the phase characteristics 
should be reasonably linear throughout the range of 
interest, 10 to 350 c. The frequency fall-off outside 
of this range should then be, in general, not greater 
than 6 db per octave. For the plugs used, the ampli¬ 
fiers were successful. 

After amplification the output pulse passes 


through a rectifier which is connected by means of 
a second cable to the recorder section, located in 
the scanning head. The main power supply was a 
24-v set of storage batteries which were recharged 
after 10 hours of operation from 115-v a-c or d-c 
sources. A third cable delivered the power to the 
scanning motor, to the sight when used, and also 
fed the output of the overall channel to the re¬ 
corder. 

Mounted and separately shielded on the chassis 
of unit B were batteries which provided bolometer 
bias of either 400 or 600 total volts across the 
bridge, in addition to filament and plate batteries 
for the preamplifier and bias batteries for the last 
two tubes in unit B. The titles on the remaining 
switches are largely self-explanatory. The cover of 
unit B was so designed that it cannot be closed 
unless all switches providing power are turned off. 

Amplifier noises were reduced sufficiently by care¬ 
ful shielding and judicious selection of vacuum 
tubes and parts to make the Johnson noise in the 
bolometer bridge the limiting factor which deter¬ 
mined the noise level. 

Indicating Unit 

The recorder is attached to the base of the scan¬ 
ning mechanism cam assembly at the rear. The sig¬ 
nal patterns are recorded on wax paper which is 
drawn from a storage reel at the center of the as¬ 
sembly. A cylindrical platen at the top of the as¬ 
sembly provides the writing surface for the record¬ 
ing stylus which is driven back and forth across 
the paper by a trolley mechanism. 

954 Field Tests for SND 

The Fort Belvoir Tests 

On the nights of January 14 to 16, 1945, the SND 
equipment was set up on a hill at Fort Belvoir for 
field tests in cooperation with the Engineer Board. 
On January 15, when a light rain was falling, 
an M2A4 and an M3A1 tank were easily detected 
at a range of 1,100 yards. A record of this test is 
given in Figure 11. Weather conditions of the kind 
prevailing on January 15 give a rather uniform 
background and it may be seen in the record that 
there is only one other significant signal and this 
was due to the top of a house. Each tank could still 
be distinctly recorded for a period of % hour after 
the engine had been shut off. 



FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


298 



Figure 11. A record of Fort Belvoir tests of SND. 


nrcnrrmflgre fY 










SCANNING INFRARED DETECTOR [SND] 


299 


On a clear night the movements of the M2A4 
and M3A1 tanks were recorded distinctly up to 
ranges of 1,950 and 1,800 yards respectively. 
Houses, roads, and trees gave much weaker signals 
than did the tanks at the same range. The rear and 
sides of each tank gave considerably stronger sig¬ 
nals than the front. During these tests the move¬ 
ment of a 14-ton Army truck and a police car 
were distinctly recorded at ranges of about 450 
yards. 

Following the January 14 to 16 field tests at Fort 
Bel voir, the passband of the amplifier was modified 
to obtain a greater signal-to-instrumental noise 
ratio. Further field tests of this model were carried 
out on the night of February 19, 1945, at Fort 
Belvoir in cooperation with the Engineer Board. 
The movements of a medium tank were recorded 
up to a maximum range of 1,800 yards. However, 
strong signals were obtained from objects in the 
terrain other than tanks. This was undoubtedly due 
to the fact that the night was cold and clear and 
that there were patches of snow on the ground. As 
a result of the amplifier modifications referred to 
above, these strong signals gave long dashes on the 
recorder chart which interfered with the resolution 
of the signals from two objects and rendered the 
interpretation of the record more difficult. 

Cape Henlopen Tests 

In tests at Cape Henlopen on the nights of 
March 23 and 24, 1945, in clear weather at 49 F 
ambient temperature and 70 per cent relative hu¬ 
midity, the SND, land-based at an elevation of 80 
feet above sea level, clearly detected the anchored 
lightship Overfalls at a range of 7,300 yards, the 
moving pilot boat at 6,000 to 8,000 yards, and an 
incoming freighter or tanker at 11,500 yards when 
it entered the scanned sector. Targets 0.25 degree 
apart could be resolved. These tests showed the 
desirability for possible ship detection purposes of 
so choosing the amplifier passband that gradual 
changes in temperature over the sea background 
would not be recorded. 

Upon the basis of the experience at Fort Belvoir 
on February 19 and at Cape Henlopen on March 
23-24 a study of the passband of the amplifier was 
made to obtain the best engineering compromise 
between the signal-to-instrumental noise ratio on 
the one hand and the best resolving power and sim¬ 
plicity of record on the other hand. In this investi¬ 


gation a variety of passbands in the amplifier and 
of input signals were used. It was tentatively con¬ 
cluded that amplifier characteristics were depend¬ 
ent upon the type of terrain being scanned and 
upon the type of record desired. In order to record 
clearly signals from small hot targets located in 
a terrain of gradually changing temperature, it was 
found desirable to design the amplifier so that the 
output signal would be proportional to the second 
derivative of the temperature with respect to dis¬ 
tance or time. The circuit constants of the SND 
amplifier were modified to accomplish this. Figure 
12 is a representative record of the results obtained 
at Cape Henlopen and was made as the SND 
scanned the horizon to detect ships. 

Airborne Tests 

The SND was tested in a PBY4 plane at the 
Naval Air Station, Quonset, Rhode Island, on the 
afternoon of March 19, 1945, primarily to deter¬ 
mine whether it would function satisfactorily when 
airborne. The test showed successful airborne oper¬ 
ation with an increase of not more than 20 per cent 
in the instrumental noise in comparison with ground 
operation. With the equipment pointed out of a 
side hatch on the plane and automatically scanning, 
each of two destroyers gave a strong record at a 
range of 3 miles and a surfaced submarine gave a 
weak signal at the same range. The shore line of 
Block Island gave a very strong signal, while lakes 
and land targets on Block Island gave moderately 
strong signals. With the scanning mechanism locked 
so that only the movement of the plane provided 
unidirectional scanning, the recorder was seen to 
operate when the line of sight crossed such targets 
as shore line, lakes, and land targets. 

From April 19 to 27, 1945, four daylight flights 
and one night flight were made with the SND in¬ 
stalled behind a silver chloride window coated with 
silver sulfide and located in the nose of an AT-11 
aircraft furnished by Wright Field and based at 
Newark Airport. The airspeed of the plane was 150 
miles per hour and the flights were made chiefly at 
an altitude of 1,000 feet, with some at 2,000 to 
4,000 feet. With the angle of view of the SND 
pointed 15 degrees below the horizontal and with 
a sector 30 degrees wide and 1.1 degrees high 
scanned twice a second, strip records of the temper¬ 
ature nonuniformities on the ground were obtained. 
Figure 13 is representative of the results obtained 




300 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 



Figure 12. A record of Cape Henlopen tests of SND 










SCANNING INFRARED DETECTOR [SND] 


301 



Figure 13. A survey of the Lambertviile-New Hope area made with SND 











302 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


in the daytime over the Lambertville-New Hope 
area of New Jersey. The populated portion is seen 
to be full of temperature boundaries and the banks 
of the Delaware River also appear in the record. 

At a later date more night flights were made at 
higher altitudes. 11 In these tests the SND scanned 
ahead from the same airplane at about 45 degrees 
from the horizontal. The airspeed was again about 


IV 


FIELD RECORD : NIGHT FLIGHT ■ Ba:389754 

SND-! i-*. AT H PLMEF 'Issue J, 10-5-45 

-a <5,v.i 


c- 




Pi if 


* 

\ 


K \ : ' . ■ 

c fe— av: 
rt. ‘■'v.y/ •'.% 

tt- 

• .•..."; 
l\ 




s . % 
w\ ? 

% . * 
. 


3 ooojf 


v 

: : • • 


g 

& 


10 - 0 ' 


vI;. • •• v W V/ fl«M . •! 

Tv-' — •* i *- ..*«*- L*. 

tlafta ^ ^ r i - • - j 

Figure 14. A record of metropolitan New York area 
made from a plane with SND. 


150 miles per hour, but the altitude for the repre¬ 
sentative record of Figure 14 was 8,000 feet. This 
record was obtained when flying south over the 
New York metropolitan area in the region of the 
East River. The edges of prominent land masses 
appear in the record as temperature boundaries. 
This mapping was done with a bolometer having a 
single exposed strip. A three-lobed pulse was ob¬ 


tained by the use of an appropriate plug. Half¬ 
wave rectification of the center lobe only tends to 
give increased resolution of the various temperature 
boundaries, as compared with Figure 13 where full- 
wave rectification was involved because of the form 
of the bolometer used. 

The results of these flights showed that the instru¬ 
ment noise was increased only about 20 per cent, 
because the SND was airborne, and that the installa¬ 
tion (SND plus window) had an overall usable sensi¬ 
tivity of 6 X 10~ 9 watt per square centimeter from 
a point source incident on the reflector. In open 
country in the daytime, signals were received only 
from buildings, paved highways and railroads; the 
signal level was greatly reduced at night. Towns 
and centers of population gave numerous strong 
signals during the day, with large details, such as a 
river through a town, clearly defined, but they gave 
somewhat smaller signals at night. Operating fac¬ 
tories with internal sources of heat gave very 
strong signals both day and night. 

955 Present Status 

Because of the field test results obtained with the 
SND, both airborne and land-based, the Army 
Board of Engineers has placed a pilot production 
order for scanner and recorder units, designed with 
the consultation of those working under Contract 
OEMsr-636 upon the basis of units used in the 
SND. These will be incorporated with Penrod, the 
Army model of the PND equipment described in 
Section 9.4, to provide a scanning receiver with 
recorder having the essential features of the SND. 

96 THERMAL MAP RECORDER 

FOR GROUND SURVEY 

961 Introduction 

The thermal map recorder for ground survey 
[TMR] 12 was developed, constructed, and flight- 
tested by Bell Telephone Laboratories under Con¬ 
tract OEMsr-636, at the request of the Army Air 
Forces as Project Controls AC-87 and AC-225.02. 
Under these control numbers were outlined the mil¬ 
itary characteristics desired, namely, to provide a 
scanning thermal receiver w T ith recorder for use in 
aircraft to plot a thermal map for the location of 
suitable ground targets for heat-homing bombs. 






303 


THERMAL MAP RECORDER FOR GROUND SURVEY 


Such a device might also find use in general stra¬ 
tegic bombing by revealing and plotting the loca¬ 
tion of hidden factories, power plants, and trans¬ 
portation routes, thus furnishing part of the infor¬ 
mation required in connection with the release of 
bombs of various types. 

This equipment plots a continuous strip map of 
temperature differentials or discontinuities present 
on the earth over which the plane flies. It can be 
adjusted to provide map dimensions of equal length 
and breadth or in which the length is some chosen 
factor of the breadth. The forming of the map may 
be observed in flight. 

Flight tests have shown that a stabilized plat¬ 


form to reduce the mapping errors caused by the 
unsteadiness of the airplane in flight is essential 
for the formation of an accurate map. 

General Description 

The complete equipment (Figure 15), consisting 
of a parabolic focusing mirror, thermistor bolom¬ 
eter detectors, scanning mechanism, amplifier, and 
recording mechanism, is a single, self-contained 
unit, housed in an aluminum case approximately 
18x18x27.5 inches, for mounting in the bombardier’s 
compartment of a bomber-type aircraft to obtain 
the proper viewing angle. For this application the 



BATTERY CONN. 


Figure 15. Photograph of thermal map recorder. 


PREAMP 

RECORDING STYLUS 
PAPER 


SWITCH 


PAPER SPEED 


MIRROR 
TILT 


TILT LOCK 


SENS 

BIAS 










304 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


case is cradled upon a floor-mounted framework by 
means of antivibration supports. The case is ar¬ 
ranged to be readily removable from its supporting 
framework for convenience in servicing. When 
mounted in the framework, the bottom of the 
equipment is approximately 6 inches above the 
floor. The total weight with mounting is approxi¬ 
mately 85 pounds and the operation is from the 
27-volt plane battery. 

A 12-inch parabolic mirror is mounted in the nose 
of the plane, behind a suitable fixed window, capa¬ 
ble of transmitting far infrared radiation. The 
mirror is scanned over a horizontal angle of 20 
degrees at a rate of 60 or 120 degrees per second. 
Two sets of thermistor bolometer elements, mounted 
at the focus of the mirror, permit a choice of verti¬ 
cal field of view of 0.22 or 0.9 degree. The scanning 
mechanism can be tilted to scan at any angle from 
10 degrees to 60 degrees below the horizontal. 

A change in temperature of the bolometer pro¬ 
duced by a change in the average temperature of 
the ground area imaged upon it produces a change 
in the bolometer resistance, and, therefore, by vir¬ 
tue of the resulting change in the d-c current flow¬ 
ing through the bolometer, is converted into a volt¬ 
age signal. These voltage signals pass through an 
amplifier of special characteristics and are applied 
to a stylus which bears on a chemically treated 
paper chart 6 inches wide. The stylus is actuated 
by other means to move back and forth across the 
chart in synchronism with the scanning mirror. 
The amplified signals cause the stylus to plot on the 
advancing paper chart a map of the temperature 
differentials of the ground areas as they are imaged 
on the bolometer. The chart is so arranged that the 
map coordinates can be adjusted for equal specific 
ground distances, depending on altitude of flight, 
speed of plane, vertical angle of view, and scan¬ 
ning rate. 

The equipment may be used to record hot areas 
against a cooler background or, by operating a 
switch, to record cold spots against a warmer back¬ 
ground. 

With the 0.22-degree vertical field bolometer and 
6 scans per second, the minimum radiant power 
from a point source of heat required to produce a 
recorded trace on the chart is 1.4 X 10" 7 watt inci¬ 
dent on the reflector. While greater sensitivity 
would be obtained at slower scanning speeds, the 
practical sensitivity for normal mapping conditions 


under which a ground area is scanned only once 
or twice would be expected to be less than this 
figure. 

9,6,3 Description of Component Parts 

Optical System 

Mirror Size and Mounting. The parabolic collect¬ 
ing mirror is a glass base surface coated with 
chrome-nickel. The mirror is 12 inches in diameter 
and 10 inches in focal length, and the circle of con¬ 
fusion is 0.4 millimeter in diameter. As shown in 
Figure 16, the mirror is supported in a gimbal-like 
mounting which permits free movement about the 
vertical axis and from 10 degrees to 60 degrees 
below the horizontal axis. 

The bolometer is mounted at the focal point of 
the mirror and is arranged for replacement if a 
change is desirable. With the bolometer at this 
point, the vertical viewing angle is 0.226 degree for 
each millimeter of length of the sensitive strip and 
the horizontal angle of view. 

Scanning and Tilt Mechanism 

The scanning motion of the mirror support is 
produced by a cam lever attached to it which rides 
on a sinusoidally shaped scanning cam driven by a 
motor through a gear-reducing mechanism. The cam 
lever is held firmly in contact with the cam by 
means of helical springs which draw against a float¬ 
ing cam lever on the opposite side of the cam. The 
mirror is scanned over a horizontal angle of 20 
degrees at a rate of about 120 degrees per second, 
and at this rate a point target would pass over the 
sensitive strip 0.2 millimeter wide in an interval 
of about Ys of the time constant of the bolometer. 
At the maximum rate of the sinusoidal scan, the 
time for one exposure will be less. It will be appar¬ 
ent, therefore, that some signal reduction results 
from 120-degree rate of scan. 

The entire mirror support and scanning drive 
mechanism is supported in turn within a casting 
which is hung on centrally located horizontal bear¬ 
ings so that the entire assembly can be tilted around 
the horizontal center axis of the mirror. Tilt is con¬ 
trolled by the positioning of two gear rack side 
members which register with spur gears on each end 
of the tilt-control transverse shaft which is located 
near the upper edge of the main amplifier. This 
shaft is rotated by a worm and wheel under man- 





thermal map recorder for ground survey 





t . 





SLIDING 

COVER 


BOLOMETER 

MOUNT 


MIRROR 


bolometer 

FOCUSING 

ADJUSTMENT 


MIRROR 
ADJUSTING 
SCREW (3) 


OSCILLATING 
FRAME 

—-TILTING 

FRAME 


SLIDING 

COVER 


305 


Figure 16. Photograph of thermal map recorder showing mirror mounting. 


ual control from the tilt-adjusting crank located 
on the top of the cabinet on the left side. An indi¬ 
cator, ranging from 10 degrees to 60 degrees below 
the horizontal, is provided for showing the setting 
of the angle of tilt and this can be checked in flight 
by means of a spirit level mounted in a calibrated 
arc scale on the right side of the cabinet near the 
top rear. 

The lateral scanning motion of the mirror in con¬ 
nection with the vertical viewing dimension of the 
bolometer results in an advancing zigzag ribbon 
pattern of coverage as the airplane moves forward. 

Motive power for scanning is furnished by a 
speed-regulated dynamotor which also generates the 
250 volts required for the vacuum-tube circuits. A 
small auxiliary permanent-magnet motor which is 
regulated centrifugally is provided for advancing the 
recording paper under the marking stylus. 


Detecting Element and Amplifier 

Detecting Element. Two pairs of quartz-backed 
thermistor strips of 4- and 1-millimeter lengths are 
furnished for respective vertical angular coverages 
of 0.9 and 0.226 degree. The strips are arranged 
vertically side by side in their mounting, each 0.2 
millimeter in width, resulting in a horizontal view¬ 
ing angle of 0.04 degree. The two strips of each 
pair are of equal length and connected in series with 
the junction between the strips brought out for con¬ 
nection to the preamplifier tube grid. The remain¬ 
ing open ends of both pairs of strips are connected 
in parallel to a balanced source of positive and nega¬ 
tive d-c bolometer voltage supply. This arrange¬ 
ment results in a minimum d-c potential at the 
center grid tap which is highly desirable to reduce 
the possibilty of noise resulting from motion of the 


























306 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


grid lead connected to the bolometer. Two bolometer 
switches are mounted on the upper rear portion of 
the mirror support ring by which either bolometer 
can be placed in service. Although the bolometer 
strips are in pairs, single strip operation is employed 
by covering over the outer strip of each pair so 
that the relative change in resistance caused by in¬ 
cident radiation will be transmitted to the amplifier. 
With the .bolometers of strip length as above, the 
total resistance presented across the high-voltage 
supply leads is about 3.5 megohms (terminals 3 and 
4). The bolometer response-time constant is about 
3 milliseconds for either pair of strips. For mapping 
from an airplane, a nose window, consisting of a 
sheet of silver chloride protected from solarization 
by a coating of gilsonite and properly braced to 
withstand air pressure, is used because of its ability 
to transmit infrared radiation. The loss in signal 
caused by this window will range from 2.5 to 
4.0 db. 

Amplifier. The amplifier consists of two parts, a 
preamplifier and a main amplifier which also con¬ 
tains a high-voltage d-c source for the bolometer 
supply. Electrically the preamplifier consists of a 
single vacuum tube and it is mounted on a bracket 
fastened to the outer stationary frame of the scan¬ 
ning mirror. The main amplifier has four tubes and 
is so designed that when used in tandem with the 
preamplifier a substantially equal response is pro¬ 
vided over a band from 40 to 400 c, the response 
at 20 c and at 800 c being approximately 6 db 
lower than the peak. The preamplifier is mounted as 
near as possible to the bolometers to reduce the 
high-impedance lead length. 

Connection between the scanning assembly and 
the stationary preamplifier is made by means of a 
spring which merely twists in torsion without chang¬ 
ing its position or straining the adjacent conductors. 
The voltage supply for the bolometer consists of 
an oscillator which generates approximately 80 kc, 
a twin diode rectifier, and a balanced condenser re¬ 
sistance network circuit for filtering and stabilizing 
the bolometer supply* one triode of the twin-triode 
tube being used as the oscillator. 

The oscillator output is impressed on the grid of 
the second triode in the tube which amplifies the 
voltage. A rectifying and filtering circuit in the tube 
output circuit produces a d-c potential across the 
bias potentiometer which has its positive end 
grounded. A maximum bias of about —120 volts is 


available from this source which supplements the 
bias of the final power tube resulting from cathode 
resistance drop. 

The power for operating the complete device is 
obtained from the airplane 27-volt storage bat¬ 
tery. 

The detecting scheme consists in changing the 
instantaneous grid potential of the preamplifier 
vacuum tube by exposing one of the bolometer strips 
of a pair to radiation collected by the parabolic 
mirror and thus causing its temperature and corre¬ 
sponding resistance to rise or fall with respect to 
the unexposed strip of the pair as the radiant* en¬ 
ergy collected varies from moment to moment. If 
the exposed strip is heated by scanning across a hot 
object, the resistance of the strip decreases rapidly. 
If the d-c bolometer supply voltage is of proper 
polarity the preamplifier grid potential rises. As 
the scan proceeds further, the hot object is passed 
and the energy falling on the exposed strip de¬ 
creases. The strip therefore cools, its resistance 
rises, and the preamplifier grid potential decreases. 
The signal thus produced represents a positive d-c 
pulse. The by-pass grid condenser and later signal¬ 
shaping circuits will not pass the d-c pulse, but 
instead pass it as an a-c signal. Only one-half of 
the a-c pulse is used in the final recording operation 
which is unidirectional. The bolometer supply volt¬ 
age, therefore, can have the polarity set so that 
either a change from cold to hot or vice versa can 
be made to record on the paper. This feature 
permits distinguishing between hot and cold ob¬ 
jects. 

Some time delay is involved between the initial 
exposure of the bolometer strip and the final re¬ 
cording pulse. This is a function of the bolometer 
time constant and the signal-shaping circuits of the 
amplifier. Since a delay occurs in each direction of 
scan relative to the true position of the signal, some 
delay correction must be introduced so that signals 
from the same target will record in proper line in 
either direction of scan. This delay correction is 
introduced in the mechanical drive of the recording 
stylus. 

Arrangements are provided for adjusting the bo¬ 
lometer voltage to the desired value for any partic¬ 
ular unit. Depending on the particular bolometer, 
the d-c supply voltage as measured to ground at 
the load side of the series resistors will usually be 
within a range of ±100 to 250 volts. 






THERMAL MAP RECORDER FOR GROUND SURVEY 


307 


Indicating Unit 

The recorder is a self-contained, easily removable 
unit, rigidly mounted upon the U-shaped cross 
member in the cabinet. The recording is done near 
the top of the inspection window and, as the map is 
plotted, the paper is drawn to the lower edge of 
the window, thus permitting the results to be seen 
immediately. A standard Sangamo recorder is em¬ 
ployed with only slight modifications of the stylus 
arrangement. 

The standard paper provides a maximum record¬ 
ing width of about 6 inches. Eosin iodide recording 
paper as originally developed for use in Navy equip¬ 
ment is used. Eosin is a fluorescent chemical which 
permits the record to be viewed clearly in invisible 
ultraviolet illumination. Notes can be made directly 
on the paper by opening the window and using an 
indelible pencil on the upper portion of the exposed 
record, which is solidly backed by the recorder case. 
The paper magazine can be recharged easily by 
removing the inspection window from the top of the 
cabinet and opening the recorder case by means of 
two levers provided for the purpose. Access to the 


take-up roll can be had by removing the upper rear 
cover plate from the cabinet. The paper record 
should always be shielded from the direct rays of 
the sun and kept in subdued light as far as pos¬ 
sible until thoroughly dry. 

The stylus of the recorder is arranged to travel 
back and forth across the paper and is driven by 
indirect mechanical connection to the mirror shaft. 
Electric signals are conducted to the stylus by a 
sliding spring contact. The lateral motion is accom¬ 
plished by translating the oscillating motion of the 
mirror through a set of gears to a shaft mounting the 
stylus drive sheave. Since the final recording signal 
is always produced slightly after the time a hot 
spot is swept over by the search beam, regardless of 
which direction the stylus is traveling, a backlash 
mechanism is included to introduce some delay in 
the position of the stylus with respect to the mirror 
position in order to get the recorded spots to line 
up properly at the same bearing as the stylus travels 
from side to side. 

A small permanent-magnet motor which is cen- 
trifugally regulated, has an adjustable speed drive 
for advancing the paper from the magazine to the 



Figure 17. Records made with TMR over Allentown vicinity. 

RE^asir.TFi) 









308 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


take-up rollers. The speed setting is under control 
of a hand-operated crank on the upper right side 
of the cabinet. An indicator is provided for showing 
the setting of the paper speed, which may be varied 
from 0.055 to 0.65 inch per second. 

9 6 4 Field Tests on the Thermal Map 
Recorder 

Flight Tests 

Airborne mapping tests were made from Newark 
Airport in an AT-11 and a B-17. The flights were 
made over the New York Harbor area; the lower 
Hudson River; the lower Raritan River; Somer- 



Figure 18. Records made with TMR over Allen¬ 
town vicinity. 


ville-Bound Brook and vicinity in New Jersey where 
a number of interesting industrial plants, highways, 
and railroads are found; Lambertville, which is a 
small town along the Delaware River; and the 
vicinity of Allentown and Bethlehem, Pennsylvania. 


Representative maps taken during these flights are 
shown in Figures 17, 18, and 19. 

The flight altitudes ranged from about 1,000 to 
15,000 feet. Speed of flight of the AT-11 ranged be¬ 
tween 150 and 160 miles per hour airspeed. Most 
flights were made between 10 A.M. and 4 P.M., but 
one was made between 9:30 and 11:15 P.M. All 
were made between April 9 and June 22, 1945. 

This rather wide variety of observations was 
taken to gain experience with the device rapidly. To 
determine optimum parameters it is best to select 
a target area and repeat runs over it under as nearly 
as possible identical conditions. This was done in 
some cases. As a result, it was noted that settings 
involving increased numbers of scans of each point 
on the ground gave better results than fewer scans, 
provided there was a margin of sensitivity to com¬ 
pensate for the longer ranges involved in the in¬ 
creased coverage. In general, higher altitudes ap¬ 
peared to give clearer maps of rivers, roads, and 
relatively large targets, although clouds were 
opaque to the wavelengths involved and had to be 
avoided. Runs over a target area in one direction 
usually resulted in a more clearly defined map than 
in the opposite direction and sometimes the re¬ 
sults were so different as to be unrecognizable. 
This is thought to be due to the effect of reflected 
sky radiation during day runs and would not be 
expected to be as noticeable at night. Initially, work 
was confined to daytime because of the greater 
facility of day flight work and to obtain photo¬ 
graphic and better visual checks on performance 
until the best operating parameters could be estab¬ 
lished. It was then planned to make a number of 
night flights to evaluate performance. This plane 
was available for only one night flight. In all flights, 
however, it was obvious that stabilization of the 
mapper would be required to obtain accurate or, 
in many cases, even recognizable mapping. 

In most cases a series of photographs of a target 
area was taken, as no one photograph gives a true 
mapping picture of the area because of the angle 
at which the photographs may be taken. Reference 
to the photostatic copies of maps of the area is 
helpful in studying the strip-map records. In most 
examples the dimensions of the strip map are fore¬ 
shortened lengthwise to aid presentation. If the 
paper speed had been stepped up, the gaps between 
scan lines would increase so that coherence of the 
target features would be lost. All strip-map records 


l^j^iTTTfT7'rrr) 





THERMAL MAP RECORDER FOR GROUND SURVEY 


309 



Figure 19. Records made with TMR over Allentown vicinity. 


taken before June 6, 1945, were at the rate of 3 
scans per second and all subsequent ones were at the 
rate of 6 scans per second, with the exception of the 
special Fort Knox tests mentioned in a later 
section. 

In general it may be stated that these flight tests 
have shown that it is possible to map important 
country highways and rivers, to distinguish between 
open country and city areas, and to recognize vari¬ 
ous localities from the record with a knowledge of 
the terrain. 

Ground Tests at Fort Knox 13 

On July 9-11 the TMR was tested at Fort Knox, 
Kentucky, in experiments with infrared detecting 


devices which might have tactical value to the U.S. 
Army Ground Forces. For these tests a special au¬ 
tomatically tilting platform was constructed and 
the scanning speed was at the rate of 3 scans per 
second. The complete device is shown in Fig¬ 
ure 20. 

The device detected personnel and hot machine 
guns near cave entrances at night at the two dis¬ 
tances tried, 300 to 400 feet and about 1,000 feet. 
This was in a denuded area but through partial 
foliage coverage in some cases. It was thought that 
one cave was detected solely as a cold spot. Move¬ 
ments of personnel could be detected readily at the 
distances employed in the denuded area and through 
partial coverage of foliage, but the device was un- 









310 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


able to detect personnel or caves in thickly foliated 
areas. Where detection was made, angular position 
relative to the observation point was obtained on 



Figure 20. Photograph of TMR used at Fort Knox. 

the record to an accuracy of a small fraction of a 
degree. Figure 21 is a sample of the records ob¬ 
tained. 

Tests made immediately afterward at Wright 
Field showed that the minimum detectable radiant 
flux density from a large radiating area was 6 X 
10“ 10 watt per square centimeter incident on the 
reflector. 

Subsequent improvements have increased the sen¬ 
sitivity considerably over the former value, a figure 
of 2 X 10~ 10 watt per square centimeter being ob¬ 
tained from a point source of fixed position relative 
to the mapper. 


Airborne Tank Detection Tests at Aberdeen 14 

On the night of July 27-28, 1945, the TMR was 
tested as an airborne tank detector at Aberdeen 
Proving Ground. For these tests the scanning speed 
was 6 scans per second and the recorded strip map 
was three inches wide. 

Tanks under way were detected and certain types 
of tanks gave good signals at ranges of 2,000 feet 
and altitudes of flight of 1,000 feet. The rear of the 
tanks gave stronger signals than the front. Not all 
types of tanks gave good signals. At greater ranges, 
signals were not satisfactory. The exposure time 
to obtain good coverage was only about 10 per cent 
of the bolometer time constant and, consequently, 
sensitivity was severely limited. A multiple ele¬ 
ment device with slower scanning rate was sug¬ 
gested as a way of improving range at some com¬ 
plication of design. 

A sample of the records obtained is shown in 
Figure 22. It is of interest to note on these records 
the trace of the adjacent roadway. This was a per¬ 
fectly straight concrete highway, yet it is diffi¬ 
cult to recognize it as such, due to the unsteadiness 
of the airplane. These were night runs, but the al¬ 
titude was only about 1,000 feet. 

Comments on Possible Future 
Developments 

Before the TMR can be considered a reasonably 
accurate airborne mapping device, a stabilized plat¬ 
form will have to be provided to neutralize un¬ 
steadiness of the plane in flight so that the position 
of the TMR may be held constant to a small frac¬ 
tion of a degree during mapping runs. 

If a larger scanning angle were employed than 
the present 20 degrees, more information would be 
crowded into the recorded map and it would be 
easier to identify targets, particularly at the lower 
altitudes. 

Flights have been made up to 15,000 feet, which 
may be low for tactical usage but is near the ceiling 
for the AT-11. It has been observed that the higher 
the altitude the steadier flight may be, but this is 
dependent upon many factors. Also night flights 
usually are much steadier than day. Under favor¬ 
able conditions at high altitudes at night, fairly 
good mapping is possible without a stabilized plat¬ 
form, but where accuracy is essential there appears 




THERMAL MAP RECORDER FOR GROUND SURVEY 


311 



Figure 21. Record obtained with TMR in Fort Knox Tests. 


to be no alternative to the use of such a plat¬ 
form. 

In general it has appeared that mapping is better 
at higher altitudes than at lower, entirely apart 
from the greater stability involved. Probably this 


is because the mapped areas are larger at higher al¬ 
titudes and so more recognizable features of the ter¬ 
rain appear on the map. Of course, cloud forma¬ 
tions between the mapping plane and earth cause 
portions of the map to be lost. Cloud formations 














312 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 







THERMAL RECEIVER WITH REMOTE INDICATOR (TYPE L) 


313 


must, therefore, be avoided. Haze merely atten¬ 
uates the signals and usually can be tolerated. 

The first group of flight tests appeared to show 
adequate sensitivity, but greater resolution seemed 
desirable. To overcome this the scanning speed was 
raised from 3 to 6 scans per second. The results 
were somewhat disappointing. At times, with 6 scans 
per second, there seemed to be inadequate sensi¬ 
tivity. It is clear that as the scanning speed is 
doubled, the exposure time, already very small 
compared with the bolometer time constant, is cut 
in half. This causes a 6-db loss because the bolom¬ 
eter resistance change can rise to only half as much 
during the target transit. Now if it is desired to 
retain the same ground coverage, the bolometer 
strip length may be cut in half. This causes a 3-db 
improvement because the noise power on the grid 
is halved, assuming, of course, that the bolometer 
supply voltage is kept constant per unit length. 
There is thus a net loss of 3 db due to doubling the 
scanning rate. 

The decibel loss for getting detail onto the record 
at twice the rate seems a small sacrifice, but the 
records seem to show more like . 6-db loss, indicat¬ 
ing that it is chiefly the large boundaries that con¬ 
tribute most to the record. If this is so, and a few 
decibels are important, then a much slower scan¬ 
ning rate should be tested to find out how much can 
be gained by improving detail through increased 
sensitivity at low scanning speeds. To obtain proper 
coverage with the slow scanning speed, either longer 
bolometers will be needed or shallower depression 
angles must be used, thus increasing the distance to 
the targets. It does not seem feasible at present to 
increase strip length much beyond 4 millimeters 
unless the width is also increased beyond the pres¬ 
ent 0.2-millimeter value, because of the physical 
fabrication problem involved. 

Reduction of the scanning speed from 120 to 15 
degrees per second, or from 6 to % scans per second, 
should increase sensitivity by a factor of nearly ten 
times, or 20 db. Maintaining coverage, however, 
will involve larger areas and point targets may not 
be detected so readily. This will tend to be an 
offsetting factor. 

Another point to consider is that at the high 
scanning speeds signals recorded from gradual 
changes in temperature from one area to adjacent 
areas are emphasized in relation to small abrupt 
thermal discontinuities. This undoubtedly results in 


some features which cause the records to be difficult 
to interpret, and lowering the scanning speed sub¬ 
stantially would reduce or eliminate this incorrect 
emphasis on gradual temperature differences in the 
scanning process. Of course, if there were sufficient 
margin of sensitivity, raising the low cutoff of the 
amplifier would reduce this effect, but at high scan¬ 
ning speeds such a margin does not exist. Reduction 
of scanning speeds has thus far been impracticable 
because of mechanical difficulties. 

Present Status 

Immediately prior to the termination of the war 
the Army Air Forces desired the TMR for further 
experimental use in connection with thermal map¬ 
ping and bombing purposes and had opened nego¬ 
tiations with a commercial manufacturer for pilot 
models. This activity has probably been temporarily 
suspended, but the final laboratory model of the 
TMR has been furnished to Wright Field for 
experimental use. 

9 7 THERMAL RECEIVER WITH 
REMOTE INDICATOR (TYPE L) 

9,7,1 Introduction 

At the request of the Navy Department, Bureau 
of Aeronautics, as Project Control NA-172, two 
models, A and B, of a scanning device employing 
thermistor bolometers suitable for installation in 
expendable radio-controlled aircraft (the drone) and 
two models of remote target bearing indicators 
suitable for use in control aircraft were developed, 
constructed, and flight-tested. These devices were 
designed to permit the guidance by remote radio 
control of expendable drones and the determination 
of their final crash dives into naval surface craft 
at night, based upon the instantaneous bearings of 
the target with respect to the drone as indicated by 
this equipment to a control operator. Transmission 
of information from the scanning unit to the indi¬ 
cator unit may be accomplished by a repeat-back 
radio link. 

As by-products of this work other basic informa¬ 
tion has been obtained which is of value to the field 
of infrared devices, particularly those using ther¬ 
mistor bolometers. This includes analyses of ther¬ 
mistor bolometer responses to rapid traverses of 





314 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


temperature boundaries; studies of amplifier design 
characteristics suitable for attainment of optimum 
signal-to-noise rendition of these responses; and de¬ 
velopment of signal treatments which provide the 
most definitive presentations of bearing information 
on cathode-ray oscilloscopes. Evaluation of the 
potentialities of infrared scanning devices employ¬ 
ing thermistor bolometers, as related to speed of 
scan, size of collecting apertures, and angular fields 
of view, provides a basis for judging the practica¬ 
bility of proposed applications generally. Field test¬ 
ing experience has added considerably to the knowl¬ 
edge of the practical limitations imposed on infra¬ 
red devices by excessive moisture, by movement of 
moist air masses across the field of view, and by 
loss of contrast between target and background 
when reflected solar energy is a masking factor. 
Airborne application has established practical re¬ 
quirements for the field of view requisite to the 
holding of a target indication in an unstabilized 
aircraft and has led to the development of shock¬ 
mounting techniques suitable for sensitive equip¬ 
ment. Considerable information relative to silver 
chloride viewing windows and their handling, surface 
treatment, and absorption losses has been gathered. 
The Model A equipment has been employed in an 
evaluation of the effect of countermeasures against 
infrared detection conducted by the Bureau of 
Ordnance in connection with an investigation of the 
possibilities of detection of the German Schnorchel 
by infrared devices for the Bureau of Aeronautics, 
and in a study of tank detection for the Army 
Engineer Board. 

Extensive airborne tests, complemented by photo¬ 
graphic evidence of the performance, have provided 
a basis for tactical evaluation. These tests have 
indicated that target bearings accurate to ±0.5 de¬ 
gree, both azimuthal and vertical, may be supplied 
for collision or homing course steering. Observed 
ranges of detection of all significant naval targets 
in 199 flight approaches have extended from a 
minimum of 2 land miles, under unfavorable day¬ 
time conditions, to a maximum of 8 land miles at 
night. 

The final report 7 under Contract OEMsr-636 
contains a full discussion of both Model A, the 
preliminary development, and Model B, the devel¬ 
opment finally subjected to exhaustive tests. Only 
Model B is considered in the description which 
follows. 


General Description 

Model B consists of two units: a scanning pickup 
and transmitting unit, and a receiving indicator unit. 
The scanner provides for simple harmonic scanning 
by a parabolic mirror 6 inches in diameter and 
4 inches in focal length, at a rate of 3 scans per 
second. Type L employs two twin-strip thermistor 
bolometers mounted in a common housing at the 
focus of the reflector so that the total vertical field 
of view is 4 degrees, comprised of two partially 
overlapping vertical zones, each representing 
slightly more than a 2-degree field of view with an 
azimuthal sweep of 50 degrees. This unit also pro¬ 
vides for the transmission of the derived signals and 
the necessary control features to a remote receiving 
and indicating unit. The indicator is a cathode-ray 
oscilloscope [CROJ the sweep of which is synchro¬ 
nized with the scanner. Hot targets relative to the 
background encountered in the upper field of view 
of the scanner are depicted as single spikes or “pips” 
above a horizontal base line; those in the lower 
field of view as pips below the line. The appearance 
of equal pips above and below signify that the 
target falls equally in the two fields of view. The 
overlapping zone is about 4 degrees. A servolink 
between the presentation unit and the scanner oper¬ 
ates a reversible motor which can tilt the optical 
axis of the scanning head from 0 to 25 degrees to 
achieve the above condition. The resulting vertical 
bearing then appears on the calibrated dial of the 
indicator unit. Azimuthal bearings of targets with 
respect to the drone are determined directly from a 
horizontal scale on the face of the CRO tube. In¬ 
formation from the scanning unit to the presenta¬ 
tion unit is relayed by a 4-channel carrier-frequency 
system. Interconnection of the two units may be 
achieved by means of two cables or by two radio 
links, one for repeat-back and the other for control. 

Physically, the scanner and its associated equip¬ 
ment have the overall form shown in Figure 23, 
which provides a right-hand view. Included in Fig¬ 
ure 23 is a front window section of the bombardier’s 
compartment of an SNB-1 aircraft, supported with 
a bracket at the proper angle and position relative 
to the scanner. This indicates the location of the 
equipment when in use. Within this section is a 
window, trapezoidal in shape, covered w T ith a 
framed sheet of silver chloride or reinforced Plio¬ 
film. 



THERMAL RECEIVER WITH REMOTE INDICATOR (TYPE L) 


315 


As may be seen from the photograph, the pickup 
and transmitting unit forms one integrated assem¬ 
bly. It is well protected against shock by a single 
aircraft-type mounting from which it may be 
quickly removed for servicing. The base dimen¬ 
sions are approximately 16x16 inches, the overall 
height 17 inches, and the complete weight, 43 
pounds. 



Figure 23. Photograph of Type L. 


Likewise, the presentation unit is antishock- 
mounted to minimize microphonic disturbances. It 
is contained in an aluminum case 10x15x20 inches 
and weighs 48 pounds. 

Power supply for the entire unit is derived from 
the 27-volt aircraft battery, the drain being less 
than 5 amperes. 

Functionally, the unit may be broken down into 
several elements: An optical and detecting system 
consisting of a parabolic reflector and a dual ther¬ 
mistor bolometer; a scanning mechanism for oscil¬ 
lating this system; narrow-band amplifiers for dif¬ 
ferentiating and amplifying the signals produced by 
the bolometer; and the transmitting portion of a 
four-channel carrier system for transmitting the 
signals pertaining to the two vertical fields of view 
and for relaying instantaneous information con¬ 
cerning the azimuth and tilt angles of the scanning 
head. In addition, there is a tilt-drive mechanism 
and power supplies for the bolometer bias and for 
the vacuum-tube requirements. 

The minimum detectable radiant power incident 
on the reflector, while scanning, is about 1.8 X 10" 7 
watt. 


9,7 3 Description of Component Parts 

Optical System 

Images of distant targets are produced by a 
parabolic reflector 7 inches in diameter having a 
focal length of 6 inches, with the bolometer mounted 
so that the sensitive elements are at the focal plane 
of this collector. On axis, the circle of confusion for 
the image of the remote point source has a diameter 
of approximately 0.7 millimeter. 

The reflector is affixed to a cast aluminum mem¬ 
ber which is mounted, gimbal-ring fashion, to pro¬ 
vide for angular movement both vertically and 
horizontally. A spun aluminum cylinder attached to 
the same member acts as a shield and a diaphragm 
to prevent entrance of energy to the reflector from 
unwanted directions. The bolometer housing is 
fitted in a focusing mount which is anchored at its 
two ends to the aluminum cylinder. 

Two pairs of thermistor bolometer strips, 0.2 
millimeter wide and 6.5 millimeters long, mounted 
parallel as indicated in Figure 24 and coextensive, 
correspond to upper and lower zones of the field of 


STRIPS 0.2 MM 
0.5 MM SEPARATION 



Figure 24. Bolometer configuration and wiring dia¬ 
gram used in Model B, Type L. 


view equivalent to 2 degrees each. A moderate over¬ 
lapping of the pairs of bolometer strips precludes 
the existence of a central “dead” zone and facilitates 
the finding of targets and their centering in the 


























316 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


overall field of view. The time constants of the 
bolometers are about 3 milliseconds. 

Scanning and Tilt Mechanism 

The simple harmonic oscillation of the scanning 
head is achieved by a two-lever mechanism moving 
along the periphery of an eccentric cam, the two 
levers being linked by springs. One lever is simply 
a follower and serves only to provide uniform pres¬ 
sure between the cam and the working lever. The 
other lever actuates the scanning head through a 
vertical shaft attached to the yoke-shaped alumi¬ 
num casting within which the optical system is 
mounted in crosswise pivots. A ball-bearing roller 
is employed as the point of contact between the 
working lever and the hardened steel cam, but for 
the sake of smooth operation a shaped block of 
oilite bronze is used with the follower lever. The 
scanning rate is about 150 degrees per second. The 
variation of scanning speed with angle in the vicin¬ 
ity of the reversal points is overcome by the pro¬ 
vision of a 50-degree scan of which only the central 
40-degree range is indexed for readings of azimuth 
on the presentation equipment. 

The driving power is derived from a speed- 
regulated dynamotor which serves also to supply a 
250-volt d-c output for plate supply to the vacuum- 
tube equipment. The dynamotor is operated on a 
standard 27-volt aircraft battery supply and rotates 
at a speed of 7,200 rpm. 

When scanning, successive images of the elements 
of the panorama are swept across the strips of the 
bolometer. In the simple case of a point discon¬ 
tinuity in the energy received from an otherwise 
uniform background, the passage of the image across 
the first of the two strips occasions a heating (or 
cooling) of the bolometer strip. The result is a 
small change in resistance and, since there is a d-c 
voltage biasing the strips, a corresponding decrease 
(or increase) occurs in the voltage drop along the 
strip. As the image passes beyond this strip, the 
resistance, and hence the voltage drop, begin to re¬ 
turn to their former values. However, as the image 
enters the second strip, similar but oppositely di¬ 
rected voltage changes occur (due to opposite polar¬ 
ity of the d-c bias). As a result, the rate of change 
of the potential at the junction of the strips is the 
sum of the changes due to the cooling of the first 
strip and the heating of the second. Finally, as the 
image emerges from the second strip, the resistances 


and voltage drops across both strips return to nor¬ 
mal. There results, therefore, an a-c signal of a 
definite characteristic spike-like waveform. Since 
on the return scan the senses of the voltage changes 
are reversed, the shape of the signal obtained is also 
reversed. When the target is nearby and large, two 
such signals are obtained, one for each bound¬ 
ary. 

The amplitude of the signal derived from the 
scanning of a target depends not alone upon the 
energy focused upon the bolometer strip as deter¬ 
mined by the size and temperature of the target, the 
diameter, focal length, and precision of the mirror, 
and the dimensions of the bolometer, but also on 
the basic sensitivity of the bolometer, the bias 
voltage, and the relationship of the bolometer time 
constant to the rate of scanning. With a 150-degree 
per second scanning rate, the time of traverse of the 
0.2-millimeter strip is of the order of 1 millisecond. 
Since the time constant of the bolometer is some¬ 
thing over 3 milliseconds, it is apparent that the 
high scanning speed is obtained at the cost of a 
substantial signal loss. This may be regained when 
bolometers of shorter time constant become avail¬ 
able. 

A tilt mechanism is provided to point the receiver 
toward its target. This mechanism consists of an 
arm attached to the pivoted optical system which 
is provided with a roller. This roller follows the 
curved slot of the cam, thereby varying the tilt of 
the scanning head. The cam shape is such that the 
rate of change of the angle of tilt increases con¬ 
tinuously as the downward tilt increases. This 
facilitates the tracking of a target during a level 
flight approach. The shape was arrived at by con¬ 
sideration of the actual rates of change in vertical 
target bearing for level flight approaches at various 
possible elevations and speeds. 

If the missile were approaching its target in level 
flight, the indicator would show a signal wholly or 
predominantly below the horizontal line in the 
observing screen. The scanner would, therefore, 
have to be tilted downward until an equal signal 
above and below the horizontal line would be regis¬ 
tered, indicating that equal amounts of energy were 
received from the target by the upper and lower 
bolometers. If the required tilt were exceeded, the 
signal would appear predominantly in the upper 
field of view, indicating a need for an upward tilt 
of the scanning head. 


1TTA wulw ld 



THERMAL RECEIVER WITH REMOTE INDICATOR (TYPE L) 


317 


Control of the tilt is achieved by a relay system 
at the pickup unit which translates the control 
instructions to reversals of the tilt-drive motor. 

Amplifiers and Carrier System 

When dual bolometers corresponding to two sep¬ 
arate zones of the field of view are used, the suc¬ 
ceeding electronic equipment must also be in dupli¬ 
cate, providing two distinct signal channels. In each 
of these channels there are 3 stages of amplification 
preceding the modulators which form a part of the 
carrier system which permits a single radio path to 
link the transmitting and receiving unit of the 
equipment. 

The amplifier for each channel consists of two 
sections, a preamplifier and a signal amplifier. The 
preamplifier is a single-stage type and is located 
close to the bolometer in a housing on top of the 
cylindrical shield of the optical system. Hearing-aid 
tubes, selected to minimize microphonic disturb¬ 
ances, are used in the preamplifier. 

Preamplifier components are rigidly fixed to an 
aluminum chassis which is antishock-mounted on 
sponge rubber. The period of this mounting is suffi¬ 
ciently low to avoid microphonic noise contribu¬ 
tions by the amplifier tubes. Connections from the 
amplifier panel to the bolometer leads and to the 
other outgoing circuits are by means of short flex¬ 
ible links of multistranded wire. Arranged to have 
very low capacitance to adjacent grounded surfaces, 
these links do not create noise. Overall, the micro¬ 
phonic response of the system to the periodic vibra¬ 
tion of the scanning mechanism should be no more 
than barely detectable in the thermal noise pattern 
as viewed at the output of the amplifier with an 
oscilloscope. 

Since the thermistor bolometer is a high-imped¬ 
ance device, the paralleled resistance of the two 
strips of one of the bolometers in this equipment 
being of the order of 4 megohms, it is important 
that the input impedance of the amplifier be high 
if signal attenuation is to be avoided. To insure 
against the flow of grid current and the reduction of 
this high-input impedance, a 1-volt Mallory bias 
cell is employed. (The use of the bias cell has 
avoided a small amount of further development 
effort. It undoubtedly could be eliminated with 
further work.) Use of relatively large grid blocking 
condensers (0.01 millifarad) assures that the input 
resistance is essentially that of the paralleled 


bolometer and grid leak resistances, thereby render¬ 
ing the thermal-noise threshold a minimum. 

The preamplifier is linked by flexible cables to 
the stationary panel on which the remainder of the 
electronic equipment, including the signal amplifier, 
is located. This amplifier is a conventional two- 
stage amplifier using type 6SJ7 tubes. 

Flexible cables link the preamplifiers through a 
plug-in connection to the stationary panel on which 
the remainder of the electronic equipment is 
mounted. Second and third stages of the signal 
amplification are mounted at the top of this panel 
and employ type 6SJ7 tubes. The choice of the grid 
coupling condensers for the second stages, however, 
is such as to provide a differentiation of the signals. 

The differentiated signals are characterized by 
prominent and symmetrical pips corresponding to 
the crossovers of target images from one to the 
other of the twin bolometer strips. It is these pips 
which are ultimately employed in the presentation 
of target bearings on the indicator unit. 

High-frequency response of the signal amplifiers 
is restricted by the use of shunt capacitances. The 
cutoff is sufficiently high to prevent appreciable 
signal attenuation, but the higher frequency com¬ 
ponents of the thermal noise background are elimi¬ 
nated, thereby enhancing the signal-to-noise ratio. 
The overall frequency response, as determined by 
the differentiating elements and the shunt capaci¬ 
tances, is such that peak gain occurs at 80 cycles, 
the gain being 6 db lower at about 20 and 200 
cycles. 

Gain control potentiometers are provided between 
the second and third stages of the amplifier to 
permit adjustment of the two-channel gains to the 
same value. Overloading of the system by the huge 
input signals of nearby targets is prevented from 
blocking the final amplifiers by the inclusion of 
5.6-megohm resistors in series with the grids. A 
similar provision is made for the grids of the modu¬ 
lators. Overloading of the radio link, which would 
result in interchannel modulation, is precluded by a 
limiting action in the signal channel modulators. 

In order that a single radio link on a single pair 
of wires may convey all the necessary information 
from the scanning device to the presentation equip¬ 
ment, four carrier frequencies are employed, 21, 17, 
13, and 9 kc. These frequencies correspond respec¬ 
tively to the signals produced by the lower and 
upper bolometers, the azimuthal position of the 



318 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


scanning head and the vertical tilt of the scanner. 
A four-channel band-pass filter couples the chan¬ 
nels to the outgoing line, effecting, at the same time, 
an impedance transformation from a 100,000-ohm 
to a 50-ohm level. 

The carrier frequencies are generated by conven¬ 
tional LC oscillators, the 17- and 21-kc oscillators 
employing the separate triode units of a single tube. 
For the 9- and 13-kc oscillators, separate tubes were 
used, the two-triode units of each being joined in 
parallel. 

The modulators for the signal channels are two- 
grid tubes where the signal frequency is introduced 
to the first of the control grids and the carrier 
frequency to the second. Since, with this type of 
modulator, there is a translation gain, these tubes 
may be considered also as providing a part of the 
signal amplification. Modulation is principally 
downward; with sinusoidal input producing 100 per 
cent downward modulation, the upward modulation 
is about 50 per cent. Larger signals which cause 
considerable downward overmodulation do not pro¬ 
duce any appreciable increase in peak carrier out¬ 
put. This limiting action safeguards against the 
overloading of the succeeding radio link. 

Modulation of the azimuth and tilt channels is 
by means of potentiometers geared to the scanning 
yoke and the pivoted optical system, respectively. 
The voltages derived from these potentiometers and 
applied to the band-pass filter are directly indica¬ 
tive of positions. At the receiving unit, the demodu¬ 
lated output of the azimuthal channel is employed 
directly for controlling the horizontal sweep of the 
cathode-ray indicator tube which is thereby syn¬ 
chronized with the scanner. The tilt channel pro¬ 
vides one link of a servo system by which the tilt is 
controlled from the receiving position. 

Bolometer bias voltage is derived from full-wave 
rectification of the output of a high-frequency 
oscillator. A high step-up ratio between the oscil¬ 
lator coil and the pick-off coils provides a terminal 
voltage of 1,000 ±500 volts. An RC network filters 
this supply and drops the voltage to that which may 
safely be applied as bolometer voltage, i.e., about 
±350 volts. 

Presentation Unit 


weighs 48 pounds. The connecting cables are at¬ 
tached on the front panel as is the practice for air¬ 
borne equipment. The controls and adjustments 
and the viewing face of the cathode-ray tube are 
also located on the front panel. 



Figure 25. Presentation unit for Type L. 

The azimuth angle of a target signal may be read 
from a scale on the face of the cathode-ray oscillo¬ 
scope and the vertical angle of the target may be 
determined from a meter. The sweep of the cathode- 
ray tube is synchronized with the motion of the 
scanning mirror and is controlled through one of 
the four carrier channels discussed in the preced¬ 
ing section. The signals arrive at the presentation 
unit along two others of the four channels and 
manifest themselves on the cathode-ray screen in 
such a way that upper and lower fields of view 
render pips on the screen above and below the 
horizontal axis of the tube, respectively. 

9 7,4 Recent Modifications in the Type L 
Equipment and the Effects upon Sensitivity 


Figure 25 shows a view of the presentation unit 
which is contained in an aluminum case, 10x15x20 
inches long. The case is antishock-mounted and 


Repeated bolometer failures, which were seem¬ 
ingly attributable to the difficulty of fabricating 
units with relatively long and narrow strips of 





THERMAL RECEIVER WITH REMOTE INDICATOR (TYPE L) 


319 


thermistor material, led to the substitution of a new 
reflector of 4-inch focal length for the 6-inch one in 
the scanner. In addition to the one-third reduction 
in the strip lengths of the bolometer elements which 
this change permitted, the strips were increased in 
width from 0.2 to 0.3 millimeter. The changes have 
now been completed and five of the new type bolom¬ 
eters have been received. The unit has also been 
refinished with black crackle lacquer. 

The new reflector has a diameter of 6 inches 
compared to the 7-inch diameter of the former 
collecting surface. Other things being comparable, 
this would have resulted in a reduction in threshold 
sensitivity in the ratio of the areas of the two reflec¬ 
tors. Upon measurement, however, employing a 
bolometer of the original type in both cases, the 
sensitivity proved to be higher with the smaller 
reflector, indicating a considerably greater optical 
merit. The new reflector is of glass, gold-surfaced, 
while the former unit was an electro-deposited 
replica with a rhodium surface. 

Changes in the strip dimensions of the bolometers, 
which were such as to maintain nearly constant 
areas, would not be expected to introduce any 
appreciable change in sensitivity, assuming that the 
bias voltages were adjusted in the two cases for 
equal power dissipation per unit of area. Compara¬ 
tive sensitivity determinations with the old and the 
new types of bolometer, employing the new reflector 
in both cases, appear to substantiate this. The bias 
voltages applied to the two bolometers in these tests 
were not in the proportion of the strip lengths, a 
nonlinear regulation in the bias supply preventing 
the application of as high a voltage to the new unit 
as would be permissible. As a result the sensitivity 
figure with the new bolometer was lower but by 
almost the amount that would be predicted. 

The net result of the change in reflectors and in 
bolometers, with the highest conveniently available 
bias applied to the new bolometer, was a negligible 
change in sensitivity. A small increase could be 
obtained if the bias voltage were to be made higher. 
The actual sensitivity figures are found in Table 3. 

Sensitivity measurements were made as follows: 
A target, masked but for a small aperture, was 
operated at a temperature about 270 centigrade 
degrees higher than the background, so high as to 
render all incidental background discontinuities 
small by comparison, thus avoiding one of the prin¬ 
cipal difficulties of this type of measurement. The 


aperture of the scanner was then reduced by sector 
masks until the signal due to the target could just 
be detected in the random noise pattern as viewed 
on the CRO tube of the presentation unit. The area 
of the reduced aperture was measured and the 
radiant energy reaching this area from the target 
was computed. This, divided by the total area of 
the reflector, yielded the figure for the minimum 
radiation density detectable by the device as nor¬ 
mally operated without the aperture masks. The 
use of sector-shaped masks properly weighted the 
rays reflected from the various radii of the mirror, 
and so was a truly proportional sample of the 
entire reflecting surface. 


Table 3. Sensitivity data on Type L. 


Reflector 

Bolometer 

Minimum detectable 
radiation density 

Old, 7 in. diameter, 

6 in. focal length 
New, 6 in. diameter, 
4 in.focal length 
New, 6 in. diameter, 
4 in.focal length 

Old, 6.5 by 0.2 mm 
strips, 450 v bias 
Old, 6.5 by 0.2 mm 
strips, 450 v bias 
New, 4.5 by 0.3 mm 
strips, 250 v bias 

1.1 X 10 -9 watt/cm 2 

0.8 X lO' 9 watt/cm 2 

1.0 X 10' 9 watt/cm 2 


9/7 5 Field Tests of the Thermal Receiver 
with Remote Indicator 

For airborne tests of the Model B of the thermal 
receiver with remote control system, an SNB-1 air¬ 
craft, 29885, was assigned to the project by the 
Navy Bureau of Aeronautics. Installation of the 
equipment was made at the Naval Air Modification 
Unit [NAMU], Johnsville, Pennsylvania, starting 
January 11, 1945. Installation included the substi¬ 
tution of a new front nose section with a silver 
chloride insert, mounting of the scanning unit in the 
bombardier’s compartment, and the placement of 
the presentation unit in the cabin, together with the 
necessary intercabling and power supply connec¬ 
tions. 

Synchronous photography of targets and of the 
corresponding indications of the infrared equip¬ 
ment proved to be a rather intricate problem. How¬ 
ever, circuit modifications were made in the presen¬ 
tation unit itself to actuate relays which would 
trigger the cameras at each limit of the scan. 
Attached to the presentation circuit was a micro¬ 
positioner polar relay, operated in series with the 
like relays employed for reversal of pip polarity at 









320 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


RUN 6, 6-14-45, LARGE FREIGHTER 
3:25 PM, SUNNY, VIS UNLDCTKD 
AIR TEMP 94\ ABS E&MID 1*02 IN^C 



2100 yds 



500 yds 



400 yds 



250 yds 


RUN 7, 6-14-45, LARGS FREIGHTER 
3:35 PM, SUNNY, VIS UNLIMITED 
AIR TEMP 94% ABS HUMID 1.02 IN^G 



2600 yds 






350 yds 


250 yds 


Figure 26. Typical record showing performance of Type L. 


























THERMAL RECEIVER WITH REMOTE INDICATOR (TYPE L) 


321 


the limits of the scan. The contacts of this relay, 
together with a battery lead, were brought out to a 
connector on the panel to which was connected an 
auxiliary camera control box. The normally closed 
relay in this unit was released momentarily at the 
beginning of each scan, during the brief reversal of 
the polar relay, and served through its back con¬ 
tacts to trigger the two-camera solenoids. Careful 
relay adjustment was required to achieve the action 
at a proper time. For movies, power could be applied 
continuously to the camera solenoids by a switch. 

The camera in the nose was set to have its shutter 
normally closed and was opened at each trigger 
pulse for a time dependent upon the setting of the 
frame speed governor. The lens on this camera pro¬ 
vided a photographic field of view of about 27x39 
degrees, the former being closely like the vertical 
tilt range of the scanner and the latter being almost 
the scanning range. Hence, with a mounting that 
brought the top of the field of view barely above 
the horizon when in flight, the camera recorded 
positions of targets consistent with those indicated 
by the infrared equipment. 

In the camera employed for CRO photographs, 
the shutter was set to be normally open and the 
governor was adjusted for maximum speed so that 
the film transfer was made quickly. Hence this 
camera recorded on each frame all that transpired 
during a particular complete scan. A lens which 
was adjustable to focus down to less than 12 inches 
was used. 

Photographic Records of Target Approaches 

Starting at NAMU on May 14, 1945, preparations 
were made for the synchronous photography of 
targets and of the corresponding CRO indications 
of the equipment. Several techniques had to be tried 
before satisfactory results were obtained, and sev¬ 
eral flights were required to establish proper param¬ 
eters for the camera settings. The final procedure 
involved the simultaneous triggering, at the end of 
each scan of the search equipment, of a camera 
mounted in the nose of the plane and another 
focused on the CRO tube. The shutter of the nose 
camera was normally closed, that of the CRO 
camera normally open. The result was a target 
picture corresponding to the start of each scan and 
a CRO picture which recorded all target indications 
produced within that scan. 

Photographic records of more than 60 target 


approaches were made, though repeated shutter 
failures in one or the other of the cameras vitiated 
about half of these for analysis or reproduction. 
Typical of the useful records are the selections 
shown in Figures 26 to 29, inclusive, representing 
all or part of about 20 runs on random ship targets, 
nearly all of which were freighters. 

As explained in Section 9.7.3, target indications 
are indicated in the form of sharp pips on a CRO 
screen, targets in the upper half of the 4-degree 
vertical field of view being presented above the 
horizontal axis and targets in the lower half below, 
a symmetrical pip designating a target centered in 
the field of view. Tilt of the scanning head by 
remote control from the presentation unit permits 
such centering. A scale of azimuth, inscribed on the 
face of the CRO, as shown in the photographs, 
designates the target bearing within a ± 20-degree 
range. A meter adjacent to the CRO indicates the 
tilt of the scanner and hence, for centered pips, the 
vertical bearing of a target. A threshold control in 
the presentation unit permits the suppression of 
CRO indications produced by noise of lower level 
than the signals. Generous use of such suppression 
was made in the photographic runs, partly for the 
sake of clarity. With less suppression, an experi¬ 
enced operator could distinguish target indications 
well down in the noise level and hence at greater 
ranges than those photographed. For this reason, 
the conditions set up for these runs are more repre¬ 
sentative of the ranges attainable for robot-control 
devices not possessed of intelligent discrimination, 
and such was the intent. 

With the above description of the operating fea¬ 
tures of the equipment, it is believed that the photo¬ 
graphed target approaches are otherwise self- 
explanatory, visibility, time of day, and weather 
conditions having been noted for each. Sharpness of 
the angular position indications is apparent and 
rough judgment may be made of the true bearing 
accuracy by comparison of the target and CRO 
photographs, since the target camera embraced a 
horizontal field of view differing by less than 1 de¬ 
gree from that of the scanner. However, misalign¬ 
ment of the nose camera sometimes shifted its axis 
a bit to the right or left of center. The complicated 
patterns on the CRO at close approach to the target 
may be correlated in many cases with deck and 
superstructure features of the targets, and where the 
approach was oblique this may be detected in the 


—RE 









322 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 



Figure 27. Typical record showing performance of Type L. These illustrations show a fading effect in infra¬ 
red transmission. Scan by scan variations (V 3 second intervals) in the target indications due to a freighter 
1 to 2 miles distant, observed from a shore location at Cape Henlopen, 11:20 a.m., June 28, 1945 with Model 
B of Type L Nancy equipment. 



















THERMAL RECEIVER WITH REMOTE INDICATOR (TYPE L) 


323 


indication by its slanting coverage of the upper and 
lower halves of the field of view. Thus, a sort of 
crude television exists. 

Not demonstrable in a selection of 8 pictures 
from camera sequences which ran to as high as 200 
or more frames is the degree of continuity of target¬ 
bearing indications. Analysis of a number of the 
complete films has shown this to average 75 per cent 
and to be as high as 95 per cent. In operation, the 
continuity appears to be even better than this, due 
to the use of a CRO with a high-persistence screen. 
Photographs lose this illusion by reason of the 
smaller latitude of the film relative to that of the 
eye and discrimination of the film against the 
weaker green persistence pattern relative to the 
blue initial trace. In a robot device, a “clamp” 
circuit could provide the memory feature of the 
persistent CRO screen, maintaining a missile on 
course during short gaps in information, such gaps 
disappearing at the closer ranges. A robot, inci¬ 
dentally, would have achieved a better result in 
vertical control than the human operator with his 
slow reactions was capable of attaining in the sam¬ 
ples of photographed runs. 

Night Flight Experiences 

Five attempts at night flights were made from 
NAMU between June 7 and 22, a number of other 
scheduled flights having been canceled due to 
weather conditions. Without night-landing facilities 
at that station, it was necessary to land at the 
Willow Grove Naval Air Station [NAS], several 
miles removed. Not only was this cumbersome but, 
with a considerable flying distance to target areas, 
and the difficulties of locating chance targets at 
night, operations on this basis were impractical. 
Dependent only on dim running lights, a pilot could 
not distinguish a ship target at sufficient distance 
to establish a run that would serve to evaluate the 
range of the equipment. (With a 4-degree vertical 
field of view and a 40-degree sweep, the equipment 
is not of itself a primary search device.) At the 
ranges of one mile or so at which runs were usually 
started, large signals were obtained from a number 
of vessels, and, in the dusk and dawn periods when 
targets could be located, representative range per¬ 
formance was obtained. 

Following this experience, a request was made 
to the Bureau of Aeronautics for a specific target 
vessel, preferably a naval craft, to operate in a 


designated area and to bear lights adequate for dis¬ 
tinguishing its presence from an aircraft at 5 or 
more miles distance. 

Learning of a special target ship aground in 
Massachuetts Bay, the Bureau of Aeronautics ar¬ 
ranged for tests from the Squantum NAS. This ship, 
the Longstreet, was equipped with artificial deck 
heating, about half of the deck area being sur¬ 
mounted by steel plates which could be heated by 
oil burners. Used by the Bureau of Ordnance for 
high-angle approaches in tests of other infrared 
devices, this target proved unsuitable for low- 
altitude runs by reason of the horizontal disposi¬ 
tion of the heated area. As shown in Figure 28, 
close approach to the Longstreet in low-level flight 
produced large signals, much larger in fact than 
from normal targets, even long after the heat had 
been shut off. However, the signal intensity de¬ 
creased with distance at a much faster rate than in 
runs on operative vessels, due to the small vertical 
component of heated area. Hence, it was decided 
that this target was of no use for evaluation of this 
type of equipment. In deciding this, runs were made 
both by day and by night and with different 
amounts of deck heating. 

Sample photographs, Figure 28, of runs on the 
Longstreet include selections from a complete day¬ 
time run, made at 4 P.M., July 13, one and a half 
hours after heat had been shut off following the 
heating of the entire deck area. Under this condi¬ 
tion the target indications probably were due largely 
to the contrasts of reflected solar energy from the 
target and the background and the run is fairly 
comparable to many others. Target crossover pic¬ 
tures for two runs at around 3:15 P.M. on July 12, 
45 minutes after the cutoff of total deck heating, 
show much stronger signals, full threshold suppres¬ 
sion having been employed in the presentation unit. 
Samples of CRO frames (Figure 28) from two night 
runs on July 12 made around 10:15 P.M., 15 and 25 
minutes after the heating of one 8x80-foot section 
of the deck had been cut off, show the more rapid 
attenuation of signal with distance than is experi¬ 
enced with normal targets. The first run was made 
at 300 feet altitude and the second at 600 feet. 

Cape Henlopen Tests 

With the Type L equipment installed on a tower 
of the Surf Club at Cape Henlopen, Delaware, 
adjacent to a Bureau of Ships infrared test station, 




324 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 




RUN 1, 7-13-45, 4:10 Hi 


ENTIRE DECK HEATED UNTIL 2:30 FM 



2400 yds 





END OF RUN 2, 7-12-45, 3:15 Bd 
ENTIRE DECK. HEAT! D UNTIL 2:30 R£ 

SUNNY, AIR TEMP 70* AES HUMID 0.7 IN/kl 



END OF RUN 3, 7-12-45, 3:20 Hi 
CONDITIONS AS ABOVE 



CRO INDICATIONS FOR NIGHT RUNS OF 7-12-45 
8’ X 80* DECK SECTION HEATED UNTIL 10 HI 
HAZY, AIR TEMP 64*, ABS HUMID, 0.7 IN /fel 


RUN 1, 10:15 Hi RUN 3, 10:25 Hi 
300* ELEV 600’ ELEV 




2900 yds 


4700 yds 





Figure 28. Typical record showing performance of Type L in flight approaches to the Longstreet, an arti¬ 
ficially heated target. 
































THERMAL RECEIVER WITH REMOTE INDICATOR (TYPE L) 


325 


RUN 4, 6-14-45, LARGS FREIGHTER 
3 PM, SUNNY, VIS. UNLIMITED 
AIR TEMP. 94%ABS. HUMID. 1.02 IN ./tel. 




1000 yds 




500 yds 


250 yds 


RUN 5, 6-14-45, LARGE FREIGHTER 
3:15 m y SUNNY, VIS. UNLIMITED 
AIR TEMP. 94% AES. HUMID. 1.02 IN./MI. 





800 yds 


550 vds 



250 yds 


Figure 29. Typical record showing performance of Type L. 





































326 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


observations of ship traffic at the entrance to Dela¬ 
ware Bay were made for a period of three days, 
June 26 to 29, 1945. Watches until midnight on two 
evenings disclosed a complete dearth of ship traffic, 
only a small patrol boat being sighted. A lightship 
3% miles offshore, counted on as a standard night 
and day target, proved barely detectable under the 
very high conditions of absolute humidity which 
prevailed. 

Fortunately, the trip to Henlopen was not futile, 
since an opportunity was provided for verifying 
the long suspected existence of a fading effect in 
the transmission of infrared radiation. First noticed 
in shore tests at Ft. Wadsworth, Staten Island, 
N. Y., and believed to have been observed in many 
flight approaches (since changes in altitude of the 
plane did not seem to explain signal variations), 
the effect was most pronounced during a portion of 
these tests. Photographs of CRO presentations of 
target indications, Figure 29, demonstrate the fad¬ 
ing, within the time interval of a single scan (% 
second), of near full-scale signals to nothing. It is, 
of course, understood that threshold suppression 
was employed in the presentation since little noise 
pattern is apparent. Hence, the “nothing” signifies 
the decrease of a signal from far above the existing 
noise level to a magnitude within the noise range. 

Cause of the fading was attributed after the Ft. 
Wadsworth tests to either the cooling of the target 
by varying air currents or the passage between 
target and detector of air masses of varying mois¬ 
ture content. The latter now appears to be the true 
explanation. At the time of taking these photo¬ 
graphs, the absolute humidity was very high, being 
equal to about 1.1 inches of precipitable water vapor 
per mile, the target was between 1 and 2 miles dis¬ 
tant (a freighter), and a brisk but variable breeze 
was blowing. On other occasions, with equal mois¬ 
ture in the air, but without the breeze, fading was 
not observed, nor was it observed in the presence 
of wind when the absolute humidity dropped to the 
order of 0.7 inch per mile. Hence, there seems to be 
a complete circumstantial indication of the cause. 

Fading must, therefore, be considered as one of 
the obstacles to infrared detection of targets. How¬ 
ever, though annoying, it has not appeared to result 
in the sacrifice of much range of detection. With a 
high-persistence screen in the CRO, a continuity 
of signal indication is maintained during the scans 
in which signals are absent. In a robot device, a 


clamp circuit could achieve the same memory func¬ 
tion, maintaining a missile on course continuously. 

Actual measurements on the sensitivity were 
made as described in Section 9.7.4. The minimum 
detectable radiation density was found to be about 
10' 9 watt per square centimeter. 

9.8 

AN ASSESSMENT OF A FAR INFRARED 
BOMBSIGHT WITH ANGULAR RATE 
RELEASE 

Introduction 

The far infrared bombsight with angular rate 
release [FIRBARR] 15 is a combination of the 
British angular rate bombsight [BARB] and equip¬ 
ment to scan and establish a line of sight to any 
heat target by using the infrared radiation from 
the target. A 5- to 10-C differential between the 
target and background is enough to allow detection 
with this device at a distance of 1 mile. The United 
States’ version of the BARB has been coded by 
the Navy as the MK23 bombsight. 

The BTL under Contract OEMsr-636 undertook 
a preliminary study which included: 

1. A mathematical analysis to obtain a first 
approximation to the bombing accuracy of 
FIRBARR. 

2. The design and construction of the infrared 
equipment, including scanning system, thermistor 
bolometer, amplifier, and indicator. 

3. The construction of test equipment and the 
procurement of tracking error data on simulated 
MK23 and FIRBARR bombsights. 

4. An analysis of these tracking data to deter¬ 
mine the optimum smoothing time of the MK23 
release circuit in order to obtain minimum horizon¬ 
tal release errors with FIRBARR. 

The tentative design objective for FIRBARR was 
that the probable error in horizontal release range 
should be less than 70 feet with manual tracking in 
an airplane in stable flight between 50 and 1,500 
feet in altitude, and between 100 and 300 knots 
closing speed. The closing speed and the collision 
course were to be set up independent of this 
equipment. 

982 Preliminary Mathematical Analysis 

Based on a series of reports on “Low Altitude 
Bombing” prepared under Section 7.2 of NDRC, a 




FAR INFRARED BOMBSIGHT WITH ANGULAR RATE RELEASE 


327 


preliminary mathematical analysis was made which 
showed that in FIRBARR the tracking error must 
be less than 0.5 degrees in qp, the depression angle, 
in order to meet the design objective. For a manual 
MK23 bombsight having a probable error in qp of 
0.1 degree and a smoothing time of 0.5 second the 
probable error in horizontal range is 30 feet at an 
altitude of 200 feet and closing speed of 150 
knots. 

The Infrared Equipment 

The infrared equipment was designed to use the 
thermistor bolometer developed at Bell Telephone 
Laboratories under Contract OEMsr-636. The par¬ 
ticular problem in FIRBARR was to design an 
amplifier, scanning system, and indicator which 
would allow the bombardier to track ships at sea to 
not more than 0.25 degree probable error in qp under 
low-level bombing conditions from ranges of 1 or 
2 miles. 

The amplifier and the input circuit connecting the 
thermistor bolometer to it were designed with the 
object of maximizing the resolving power and sensi¬ 
tivity of the system. A detailed discussion of the 
amplifier circuit design is given in Appendix A of 
the contractors’ report. 15 A description of the physi¬ 
cal construction and electrical characteristics is 
given in Appendix B of the same report. 15 

The collecting mirror in the scanning system is 
parabolic with the thermistor placed in the focal 
plane to permit a vertical scan of 6 degrees with a 
beam 3 degrees wide. Three separate mechanical 
systems were tried, but an offset rotating parabolic 
mirror mechanism w T as tentatively decided upon 
because of its simplicity and its potential ability to 
furnish the tilt and azimuth angles of the line of 
sight with a single mirror and amplifier. 

Two different types of indicating systems were 
tried. The strobotron type is preferable to the oscil¬ 
loscope for airborne equipment, since it is smaller, 
lighter in weight, and requires less electrical power 
to operate. It has the further advantage that it 
permits the identification of w T eaker infrared signals 
against the noise background by a ratio of at least 
4 to 1. 

Using a 3-inch diameter parabolic mirror having 
a 2-inch focal length, the offset rotating parabolic 
mirror scanning system, and the strobotron indi¬ 
cator, it is possible to track an infrared point target, 
the radiant energy density of which at the mirror 


surface is 5 X 10~ 10 watt per square centimeter. 
(This is total radiation without a filter.) 

9,8,3 Test Equipment 

In order to study the tracking errors in manu¬ 
ally operated MK23 and FIRBARR bombsights, 
a servo-controlled “running rabbit” was built to pro¬ 
vide either an optical or infrared line-of-sight equiv¬ 
alent to that from an airplane pursuing a collision 
course at constant altitude and airspeed over a ship 
at sea. This testing equipment provides a continu¬ 
ous photographic record (16-millimeter Fairchild 
motion-picture gun camera) of the tracking errors 
during the bombing run as well as a record (stylus 
on waxed paper) of the depression angle at release, 
both of which are accurate to better than 0.05 de¬ 
gree. In accordance with appropriate dial settings 
the servo automatically provides the line of sight 
during the 10 seconds prior to release for altitudes 
between 50 and 1,500 feet and closing speeds be¬ 
tween 80 and 350 knots. 

9 8 4 Analysis of Tracking Errors 

Using the test equipment described above, a num¬ 
ber of tracking error records were made during 
simulated bombing runs with a FIRBARR 
(mounted on a firm platform), including an oscil¬ 
lating scanning mirror and an oscilloscope indicator. 
Assuming an error of 25 feet in the determination 
of the altitude of the plane and 0.5-second expo¬ 
nential smoothing time in the BARB release 
mechanism, a probable horizontal release error 
of 50 feet is obtained. By increasing the smoothing 
time to 1.5 seconds, the probable horizontal release 
error is reduced to 32 feet. Since the theoretical 
improvement resulting from a further increase in 
the time constant is small and the equipment 
arrangement becomes difficult, 1.5 seconds has been 
chosen as the optimum smoothing time for the 
BARB sight with infrared scanning. 

985 Present Status 

The results of this investigation were sufficiently 
promising to warrant further development under 
Navy (Bureau of Ordnance) contract. The design 
objective is a bombsight incorporating infrared 
scanning and manual tracking which, when oper- 



328 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


ated in an airplane in stable flight between 50 and 
500 feet altitude at 100 to 250 knots ground speed, 
will have a probable error in horizontal release 
range of 50 feet or less. 

99 THE PORTABLE SHIP DETECTOR 
[PSD] 

9.9.1 Introduction 

The pressing need for a device that could be 
used on the deck of a submarine without revealing 
its presence to detect an enemy craft at night and 
to determine its bearing led the Navy to request, as 
Project Control NS-121, the development of a port¬ 
able infrared receiver for this purpose. The limita¬ 
tion to location on the deck of a submarine was later 
lifted by the Services. In addition the project was 
expanded as Project Control N-108 to include the 
detection of night landing parties, including (1) 
enemy transports or other large vessels at consid¬ 
erable distances, (2) landing barges and similar size 
vessels at 1- or 2-mile ranges, (3) small boats with¬ 
out engines carrying a few men at ranges up to a 
mile or more, by the infrared radiation emitted by 
them. It was desired by the Services that the equip¬ 
ment developed should be simple and easy to manu¬ 
facture and operate. 

9.9.2 General Description 

The portable ship detector [PSD] 16 operates 
solely from the difference between the amount of 
heat received from the target and that received from 
an equal adjacent solid angle of background. The 
device contains two heat-sensitive thermistor de¬ 
tecting strips which are mounted in the focal plane 
of a Schmidt optical system. A motor oscillates the 
optical system through a small angle so that the 
image of the target falls alternately on the two 
thermistor strips. The small temperature changes 
which occur in the bolometer strips when a heat 
source enters the field of view are detected elec¬ 
trically and when amplified manifest themselves as 
audible or visual signals occurring at the scanning 
frequency. The signals yield “right and left” infor¬ 
mation so that the PSD may be adapted to auto¬ 
matic following. 

As developed, the telescope unit, containing the 
optical system and the preamplifier, could be hand- 


pointed or supported by a pivot mounting on a 
tripod. The minimum detectable power density inci¬ 
dent on the optical system was determined to be 
3.6 X 10" 10 watt per square centimeter. The com¬ 
plete equipment is shown in Figure 30. 

9 9 3 Description of Component Parts 

Optical System 

Except for an external window, the entire infra¬ 
red optical system is housed in a metal cell which 
is about 2.5 inches in diameter and 3 inches long. 
The radiation receivers are mounted in the focal 
plane of a Schmidt optical system which consists of 
a spherical mirror and a rock-salt correcting plate, 
the latter serving also as the cell window. The 
aperture of the system is 34 millimeters and the 
focal length is 22 millimeters; thus the speed is 
about //0.65. The radiation detectors are about 
0.2x1.0 millimeter and the space between them is 
approximately equal to the width of one of the 
detectors. Schmidt correcting plates, which have 
sufficient accuracy so that the circle of confusion is 
small when compared to the width of the detector 
strip, may be turned on a lathe. 

The PSD thus has two fields of view which are 
rectangular corresponding to the images formed in 
the two detecting elements by a target. The field of 
view has been kept as small as possible for two 
reasons, namely, the minimum detectable signal, 
which is a function of the noise in the receiver, 
varies approximately with the square root of the 
area of the detector, and the noise and spurious 
signals due to thermal background increase rapidly 
as the vertical field is increased. The ideal condi¬ 
tion would be to have the size of the image the same 
as the size of the detector, but reduction in the size 
of the field of view increases the difficulty in locating 
a target or following a target once it has been 
found. A field of view which subtends a vertical 
angle of 2.5 degrees and a horizontal angle of 0.5 
degree was finally adopted. 

Scanning System 

The optical system, the preamplifier and the 
bolometer bridge are housed together in a small 
aluminum box which is roughly thermostated at a 
temperature above the highest expected ambient 
temperature. The aluminum box has a rock-salt 
window and forms what is known as the telescope 





THE PORTABLE SHIP DETECTOR [PSD] 


329 



Figure 30. Photograph of portable ship detector. 


unit. This box is mounted on pivots and the small 
angle of scan of the optical system is obtained by 
means of a cam driven by a small synchronous 
motor. 

The scanning rate was chosen to yield an opti¬ 
mum performance, i.e., it must be slow enough to 
give the desired sensitivity yet fast enough to main¬ 
tain a satisfactory flow of information. The char¬ 
acteristics of the thermistor elements adapt them¬ 
selves well to a rate of oscillation of from 2 to 4 c. 
A rate of oscillation of the optical system of 2 c was 
finally adopted and at this rate four signals per 
second occur when a target occurs in the center of 
the field. 

Detecting Element and Amplifier 

Detecting Element. The PSD employs an air¬ 
cooled thermistor bolometer, as described in Chap¬ 
ter 8, as its detecting element. 

At the scanning rate adopted for the PSD, using 
detecting strips equivalent to a horizontal field of 
view of 0.5 degree and separated by a distance 
equivalent to 0.5 degree, the exposure time is about 
% second. 

A reasonable match was realized by selecting 
thermistor strips which had a time constant of 120 


milliseconds. The strips were then about 16 \i thick 
and were operated in air at a pressure of about 
7 millimeters (Hg). At this pressure no gas- 
microphonic difficulties were encountered. 

It should be recalled from Chapter 8 that elec¬ 
trically the thermistor bolometer is a high-resist¬ 
ance circuit element which has a large negative 
temperature coefficient of resistance. A receiver 
which is 0.2 millimeter wide and 1.0 millimeter 
long has a cold resistance of about one megohm. 
Because of the negative temperature coefficient of 
the material, there is a certain maximum voltage 
which may be maintained across the unit with a 
corresponding current. For greater current in a unit, 
a ballast resistor must be used. For the receiver 
described previously, the peak voltage is about 35.5 
volts. It was found experimentally that for the PSD 
equipment the best signal-to-noise ratio was ordi¬ 
narily obtained when the voltage across the bolom¬ 
eter was about 20 volts. 

Bolometer Bridge and Amplifier. The thermistor 
bolometers are made with two arms and two 
wire-wound resistors, the remaining two arms of 
a Wheatstone bridge across which is applied an 
alternation potential of 1,000 c from a specially 
shielded oscillator. Means are provided for estab- 










330 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


lishing resistive and capacitive balances. The out¬ 
put of the bridge is led directly into the preamplifier 
which consists of a single pentode stage with a 
cathode follower. This network feeds into a low- 
impedance line. 

The signal from the preamplifier passes through 
a detecting unit which produces a rectified envelope 
of the signal, modulated by the variation in the 
resistive balance of the bolometer bridge. The final 
amplifier is a narrow passband amplifier of fre¬ 
quency width 2 c, built from simple RC filters, and 
has an overall gain of about 3 X 10 G . 

Power Supply 

The power supply is a conventional regulated unit 
of the degenerative type. There is some forward¬ 
acting compensation against line voltage changes. 
It operates satisfactorily on line voltages from 105 
to 125 volts. More recently a power supply has been 
designed for use with the stabilized ship detector 
[SSD] (see Section 9.10) which is superior to the 
one described here. 

Presentation Unit 

In order to provide quantitative measurement of 
the signal strength, a zero-center milliammeter is 
used to indicate the difference between the plate 
currents of the two output tubes. A signal may be 
observed visually by blinking lights which are oper¬ 
ated by relays in the plate circuits of the output 
tubes. The output stage is, in addition, used to 
operate a dual “magic eye.” The relays also control 
the output of oscillators which have frequencies in 
the audible ranges so that the signal may be de¬ 
tected by the use of headphones. The output of the 
amplifiers also operates relays which give “right” 
and “left” indications. 

It is desirable to eliminate those signals which 
occur when the heat image is scanning only across 
one strip (off-center scanning). The phase angle of 
the low-frequency output signal relative to the 
mechanical phase angle of the telescope or scanner 
depends not only upon the amplifier characteristics 
but also upon the position of the heat image as it 
scans across the strips. When the telescope is 
pointed so that the heat image falls periodically on 
only one bolometer strip the output signal differs in 
phase by 180 degrees from the case when the heat 
image falls alternately on the two strips. It is obvi¬ 
ously desirable to be able to distinguish between 


the on-center signal and the right and left off-center 
signals. The on-center signal has approximately 
twice the amplitude of the others, but in detecting 
a target of unknown strength either a comparison 
of two signals strengths would have to be made or 
the three signals would have to be located and the 
center one chosen. The 180-degree change in phase 
angle makes it possible to pick the center signal 
easily and unambiguously and incidentally to in¬ 
crease the discrimination against unwanted signals. 

A mechanical switch operated by a cam which is 
driven by the scanning motor is used to short to 
ground alternately the two sides of the balanced 
low-frequency amplifier. This scheme has been 
called a “beep-canceler,” since it removes unwanted 
audio signals. 

In the case of night detection of naval targets, 
the target is warmer than the background during 
all seasons. The beep-canceler sets the condition that 
the relays may operate only when the image of a 
hot target is scanned on-center. Hence the relays 
operate alternately, giving the high and low pitch 
sounds, and open the magic-eye shadows. For off- 
center scanning the negative pulses do not operate 
the relays, so no audible signal is given, but the 
magic-eye shadows close alternately, indicating that 
there is a target a degree or two off center. 

The thermal background may have hot or cold 
spots. The beep-canceler, of course, can do nothing 
to distinguish between a target and a hot spot in 
the thermal background. Off-center scanning of a 
cold spot will produce relay signals, while on-center 
scanning will not cause relay operation; for the 
on-center case, the negative signals will appear on 
the magic eye. 

In some instances it was desirable to use a record¬ 
ing output meter; in this case the beep-canceler 
was used as a synchronous rectifier. 

Range and Sensitivity 

Minimum Detectable Signal 

For field operation, the limitation on the sensi¬ 
tivity is generally fixed by the thermal background 
conditions. To test the sensitivity of the device 
working against a perfect background, i.e., to deter¬ 
mine the limit of sensitivity due to circuit noise, the 
following method was employed. A black-body 
source which is about 10 to 25 C above room tem¬ 
perature and subtending about 7 minutes 2 is used 






THE PORTABLE SHIP DETECTOR [PSD] 


331 


and the minimum gain of the amplifier is found for 
which relay operation is reliable. A cover is then 
placed over the cell and with the same scanning 
motion the gain of the amplifier is increased until 
20 to 30 false relay operations occur per minute. 
Since 240 relay operations occur per minute (scan- 
ing rate 2 c) when the source is reliably detected, 
the signal-to-noise ratio might be said to be about 
8 to 1. The flux incident on the apparatus is then 
computed and from the ratio of amplifier gain 
settings the minimum detectable signal may be 
obtained. In a typical trial the minimum detectable 
signal for a signal-to-noise ratio of 8 to 1 was found 
to be 3.6 X 10' 10 watt per square centimeter. When 
the scanning speed was reduced to 1 c and lower 
resistance units were used which could be operated 
at a higher fraction of their peak voltage, the mini¬ 
mum detectable signal for the same signal-to-noise 
ratio was 10' 10 watt per square centimeter. 

In making these measurements it is ordinarily 
convenient to use a somewhat larger source and 
correspondingly lower gain, since temperature gradi¬ 
ents in the screen which forms the background are 
troublesome. A correction may be readily made by 
scanning the background without the source and 
noting the amplitude and phase of the signal. 

Detection Ranges 

The PSD was first demonstrated to Section 16.4 
on May 11, 1943. In spite of rain and fog the de¬ 
stroyer Semmes with only two forestacks active was 
reliably detected at 4,100 yards. Subsequent tests 
on June 18, 1943, after further improvements had 
been made on the sensitivity, showed ranges of 
6,500 yards on ship traffic near Solomon’s Island, 
Maryland. 

The data presented in the following paragraphs 
were obtained during tests which were arranged by 
the Bureau of Ships. All ranges which are given are 
based on radar information; the aspect of the vessel 
was obtained from plots of the courses of the target 
vessel. 

1. These tests were made with the PSD and 
another far infrared detector mounted on top of 
the periscope of the USS Bass to determine the 
range and the bearing of the USS Melville and 
DE 177. The tests were carried out on December 1, 
1943, from 19:40 to 23:40 with the submarine at 
periscope depth, a position sufficiently stable for the 
successful operation of the unstabilized PSD. The 


PSD was able to determine the target bearing within 
0.5 degree. The results are stated in Table 4. 


Table 4. Results of tests made on target ships. 


Target 

Aspect bow-on 
(degrees) 

Range 

(yd) 

USS Melville (4,000 tons) 


5,700 


30 

5,500 


145 

5,300 

DE 177 


4,450 


335 

3,900 


145 

6,200 


The ranges noted are not necessarily threshold 
ranges, since only a small fraction of the available 
amplifier gain could be used because of poor ther¬ 
mal background. Because the two detectors which 
were in operation did not “look” in the same direc¬ 
tion, the periscope was trained alternately by the 
two operators. During the tests, the PSD ceased to 
function because of salt w T ater in the auxiliary unit 
which was inside the conning tower. 

On December 2 the PSD was in operating condi¬ 
tion for a continuation of the tests, which had to be 
abandoned because of a collision with the Melville 
during the first run. Strong signals preceded the 
collision. 


Table 5. Tests at the Bureau of Ships Test Station, 
Cape Henlopen, Delaware, February 23 to 24, 1944. 


Target 

Aspect 

(degrees) 

Range 

(yards) 

Time 

LCI 

90 

6,500 

19:40-04:30 


10 

6,000 

19:40-04:30 


90 

7.800 

19:40-04:30 


0 

3.900 

19:40-04:30 

LCT 

170 

5.600 

19:40-04:30 


5 

5,800 

19:40-04:30 


9 

8,600 

19:40-04:30 

Tanker 

Stern 

11,000 

19:40-04:30 

Cargo ship 

Stern 

9,000 

19:40-04:30 

LCI 

30 

7,000 

20:50-01:10 


5 

5,320 

20:50-01:10 

LCT 

210 

5,000 

20:50-01:10 

2. The 

same model of the PSD 

was land-based 


at an elevation of about 50 feet above sea level at 
Cape Henlopen for tests from February 21 to 24, 
1944. Data taken during these days on a tanker, a 
cargo vessel, an LCI, and an LCT are given in 
Table 5. 













332 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


99,5 Present Status 

Since the tests with the PSD showed that a non- 
stabilized receiver, with the requisite small angle of 
view for necessary sensitivity of detection of ship 
targets at desired ranges, could not be used satis¬ 
factorily for search purposes from a ship’s deck, this 
development as such was terminated. The results of 
this work were used, however, in the development 
of the SSD. 

9 10 THE STABILIZED SHIP DETECTOR 
[SSD] 

9101 Introduction 

Shipboard experience with the portable ship dc* 
tector [PSD] (see Section 9.9) showed that satisfac¬ 
tory rapid search for unknown targets with far 
infrared receivers was impracticable without the 
following improvements: (1) stabilization of the 
receivers; (2) elimination of manual pointing; (3) 
provision for a “target bearing memory.” The 
Bureau of Ships therefore requested, under Project 
Control No. NS-181, that a gyrostabilized, auto¬ 
matically scanning and recording thermal receiver 
incorporating the general features of the PSD, be 
developed for the detection of ships in darkness, 
smoke, or light haze conditions. The military char¬ 
acteristics desired were that the detecting element 
be mounted in a Schmidt optical system which 
would scan automatically at a fixed rate through 
a horizontal angle of 300 degrees; that the scanning 
head be mounted on a gyrostabilized platform to be 
furnished by the Navy; that the signal be amplified 
by a band-pass amplifier, the passband being chosen 
to take advantage of the fixed rate of scanning, and 
that the received signal be recorded by a suitable 
recorder. 

The signal-to-noise ratio obtainable with the 
PSD, which employed automatic scanning over a 
small horizontal angle and manual pointing for 
search purposes, was high, since with a scanning 
rate of 2 c a band-pass of two octaves is only a few 
cycles wide. The minimum detectable radiant power 
density for the PSD was about 4 X 10" 10 watt per 
square centimeter incident on the 34-millimeter 
diameter mirror for a 2-c scanning frequency (four 
signals per second). The problem in the develop¬ 
ment of the stabilized ship detector [SSD] 17 was 
to devise a receiver having sensitivity comparable 


to, or greater than, that of the PSD together with 
the additional desirable operational characteristics. 

The equipment as developed possessed this de¬ 
sired sensitivity as well as the desired operational 
features of furnishing relatively rapid search for 
good bearing accuracy and resolution of targets. A 
permanent record is made of all targets detected 
within the area scanned. 

General Description 

The complete SSD equipment consists of five 
units: (1) a scanning head, containing the two-strip 
thermistor bolometer mounted in the focal plane of 
a Schmidt optical system, and the preamplifier, 
which is mounted on a stabilized platform on the 
deck or superstructure of the ship and which may 
be adjusted to rotate through an angle of 360 de¬ 
grees or a smaller angle in the horizontal plane at 
constant angular velocity with reference to fixed 
axes; (2) a synchro-driven yaw-correction unit to 
correct the motion of the scanning head for the 
yawing of the ship and consequently to produce a 
scan relative to fixed axes rather than relative to 
the ship and to permit a record to be made of 
the true bearing of the target; (3) an amplifier of 
special characteristics for the bolometer output; 
(4) a signal recorder; and (5) a regulated power 
supply operating directly from the ship’s power 
lines. Units (2) to (5) may be located in the CIC 
room of the ship for convenience of operation and 
more rapid receipt of intelligence. 

The heat image of any target within the angle 
which is scanned falls successively on the two ther¬ 
mistor strips. The extremely small temperature 
changes of the heat-sensitive elements are detected 
electrically, giving rise to two electrical pulses of 
opposite polarity. The complex output pulse from 
the amplifier actuates an electrically operated re¬ 
corder which is operated in synchronism with the 
scanning head and which prints on a paper chart a 
continuous, permanent record. In the case of sector 
scanning the stylus moves back and forth across the 
recording paper, which is driven slowly. The posi¬ 
tion of the stylus at any instant corresponds to a 
certain true bearing of the scanning head. Chemical 
recording paper is used and the passage of an elec¬ 
tric pulse produces a discoloration of the paper. 
If the true bearing of a target remains constant for 
successive scans, the record consists of a series of 







THE STABILIZED SHIP DETECTOR [SSD] 


333 


spots which lie in a straight line parallel to the 
motion of the paper chart. The constant motion of 
the paper establishes a time scale, hence the record 
produced is one which gives bearing as a function 
of time. 

910,3 Description of the Component Parts 

Optical System 

Except for an external, coated silver-chloride 
window, the entire infrared optical system is housed 
in a sealed metal tube which has a rock-salt window. 
The heat sensitive detectors are mounted in the 
focal plane of a Schmidt optical system which con¬ 
sists of a spherical mirror and a rock-salt correcting 
plate which also serves as the cell window. This 
complete unit is designated as the “cell” and, in the 
latest model, has an aperture of 150 millimeters, 
which is near the maximum aperture practical with 
rock-salt optics at the present time. The cell units 
are brought into focus by placing shims between 
the bolometer housing and the fixture on the post 
supporting the bolometer. When the position for 
best focus has been determined, the cell is evacu¬ 
ated, filled with dry air at a pressure of about 
1 centimeter (Hg) and sealed off. 

It is important that the optical system have the 
maximum sensitivity for its weight and volume. 
The Schmidt optical system has therefore been 
chosen in preference to the parabolic mirror system, 
inasmuch as it is possible to demonstrate 18 that 
for equally good definition the former system may 
occupy only about % the volume of the latter sys¬ 
tem. The optical system of a carefully constructed 
SSD cell of 150 millimeters aperture has a circle 
of confusion of about 6 minutes of arc. 

The SSD has two fields of view corresponding to 
the two parallel thermistor bolometer strips each of 
which is 1.0 millimeter long and 0.2 millimeter wide. 
The strips are 0.2 millimeter apart. These strips, 
placed vertically at the principal focus of the opti¬ 
cal system with an 83-millimeter focal length, pro¬ 
duce two fields of view each 40 minutes high and 
8 minutes wide, separated by 8 minutes. 

It is desirable to keep the vertical extent of the 
field of view small in order that spurious signals be 
kept at a minimum. It is fairly obvious that the 
ideal condition would be that the size of the image 
of the target be comparable to the dimensions of 
the detector or bolometer strip. This condition may 


be approached for land-based operation but cannot 
be realized at sea because of stabilization difficulties. 
A naval target, such as a destroyer escort at 10,000 
yards, subtends a vertical angle of less than four 
minutes, whereas stabilization errors may be of the 
order of ±10 minutes. 

There are several factors which must be consid¬ 
ered in choosing the vertical extent of the field of 
view. Some of these which relate to the stabilization 
problem are: the random errors in the gyro-vertical; 
possible drifts in the mean vertical (as occur during 
rapid changes of course); ship flexure between the 
gyro-vertical and the scanning head (particularly 
for mast mounting); and inaccuracies in alignment 
which occur during installation or maintenance. 
Particularly in the case of mast mounting, small 
craft such as PT boats may escape detection if the 
vertical field is too small. Consideration of these 
factors led to the choice of a vertical field of view 
of about 40 minutes. It might be well to increase 
this to about one degree for masthead operation. 

The considerations leading to the choice of the 
width of the field of view and the separation of the 
fields of view are more complex than in the case of 
the extent of the vertical field of view. Factors to be 
considered are: desired resolution, sensitivity, back¬ 
ground noise, and the shape and duration of the 
output pulse. The fields of view which have been 
used in the SSD give reasonably good resolution; 
ordinarily, targets whose bearings differ by one 
degree or somewhat less may be resolved. Increasing 
the horizontal fields of view would obviously de¬ 
crease the resolving power of the detector. 

It is desirable that the extent of the entire heat 
image of a small target (including the circle of con¬ 
fusion) be comparable to the strip width. The opti¬ 
cal system of a carefully constructed SSD cell of 
100-millimeter aperture has a circle of confusion of 
roughly four minutes; therefore, the heat from a 
small target, such as a destroyer escort with stern 
aspect of 10,000 yards, is concentrated successively 
on the strips. 

Scanning Mechanism 

The optical unit and the d-c preamplifier are 
mounted together on an aluminum plate which is 
oscillated at a rate of 6, 12, or 18 degrees per second 
as desired, by a synchro motor through a gear train. 
The scanning mechanism provides either for unat¬ 
tended 360-degree search or, by a rapid adjustment, 





334 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


for scanning over smaller section angles of 25, 50, 
or 150 degrees. Because the SSD is to be operated 
on shipboard it is necessary that the scanning be 
with reference to fixed axes rather than relative to 
the ship, since the yawing of the ship causes poor 
alignment of the signals on the record. 

A record of the true bearing of a target was 
obtained by putting gyrocompass corrections into 
the motion of the scanning head. The drive unit 
to accomplish this contains a motor, two synchro 
generators and a Ford Instrument Company com¬ 
pass follow-up mechanism. One synchro generator 
was driven directly by the motor at a constant 
speed and was linked electrically to a synchro motor 
in the recorder to provide power for the stylus. At 
the end of each scan two phosphor-bronze fingers 
attached to the stylus carriage closed contacts in a 
relay circuit which controlled the direction of rota¬ 
tion of the driving motor. The other motor, which 
was linked electrically to a synchro motor in the 
scanning head, was driven by a gear train in which 
the gyrocompass corrections were mixed with the 
scanning drive by means of a mechanical differen¬ 
tial. 

Three dials were provided from which could be 


read the ship’s course and the train of the scanning 
head in terms of relative and true bearing. By 
means of a screw-driver adjustment of a differential, 
the position of the head might be changed to corre¬ 
spond to the true bearing indicated on the recorder. 

In Figure 31 is shown the complete equipment for 
the synchro driven system designed for operation 
from the ship’s power lines. The electronic units are 
the final amplifier (right) and the power supply for 
the bolometer and preamplifier. 

Detecting Element and Amplifiers 

Detecting Element. The SSD employs gas-cooled 
thermistor bolometers for the detection of the infra¬ 
red radiation. The thermistor strips are mounted 
on a molded glass blank which is held by a poly¬ 
styrene cup covered by a thin plane rock-salt 
window. A well is molded in the glass disk very 
nearly 1 millimeter wide and about 0.25 millimeter 
deep and the electrical contacts are made quite near 
the edges of this well. A small breather hole is 
provided so that the optical unit may be exhausted 
without damaging the detector units. The heat- 
sensitive areas are rectangular, 1.0x0.2 millimeter, 
and are separated by 0.2 millimeter. 











THE STABILIZED SHIP DETECTOR [SSD] 


335 


Electrically, each strip is a high-resistance circuit 
element with a large negative temperature coefficient 
of resistance, the resistance of a single strip at room 
temperature being about 10 megohms. A constant 
potential difference is maintained across the two 
units connected in series and the variation of poten¬ 
tial across one of the units gives an indication of 
temperature changes which occur in either unit. 
Under operating conditions the electric power dis¬ 
sipation in the units raises their temperature about 
15 centigrade degrees above the surroundings and 
the resistance of each unit decreases to about 
7 megohms. A certain maximum voltage exists which 
may safely be applied to the thermistor bolometer 
(see Chapter 8). For practical operation the maxi¬ 
mum sensitivity of the bolometer to radiation lies 
at somewhat less than peak voltage. The SSD cells 
were operated at about 80 per cent of peak voltage. 

The present thermistor bolometers show little 
evidence of increase in noise with increase in cur¬ 
rent through the units. Because of the decrease in 
resistance which occurs with increasing current in 
the units the “Johnson noise’’ should decrease with 
increasing current. Most thermistor bolometers show 
an actual decrease in noise as the current is in¬ 
creased. The condition has been set arbitrarily that 
a unit is satisfactory if the noise at 80 per cent of 
peak voltage is no greater than the noise without 
current. 

In the initial design of the SSD the shortest 
detectors which were practical to manufacture at 
the time were chosen. The desired vertical field of 
view and the length of the detectors set the focal 
length of the mirror. The width of the detector 
(0.2 millimeter) was chosen to give sufficiently good 
resolution and a reasonably short electrical output 
pulse for 6 degrees per second scanning. The first 
cells, which were of 64-millimeter aperture, formed 
images which were sharp enough to allow the use of 
the narrow detectors. Later, cells of 100-millimeter 
aperture (of the same focal length) were developed; 
these also had sufficiently good optics to perform 
efficiently. The most recent cells, of 150-millimeter 
aperture (//0.57), are not entirely satisfactory be¬ 
cause of excessive chromatic aberration. 

Amplifier. The electronic units of the SSD con¬ 
sist of the preamplifier, a power supply for the 
bolometer and a final amplifier. 

Preamplifier . The preamplifier is the most critical 
part of the electronic equipment. No satisfactory 


solution of the problem of isolating the preamplifier 
from ships’ vibrations has been found. Currently 
available methods could not be used to isolate the 
optical unit from vibration because of the necessity 
for maintaining the axis of the optical unit on the 
horizon. The entire equipment was therefore sub¬ 
jected to the ship’s vibration, but all parts were 
very rigidly mounted in order to avoid amplification 
of the vibrations. The preamplifier should operate 
satisfactorily on signals which are a little more than 
1 pv. It is desirable to use as few tubes as possible 
but to have sufficient gain that the output signal be 
large compared to any noise due to slip rings or hum 
pickup in the output cable. The preamplifier has a 
voltage gain of about 8,000 which is sufficient to 
provide about 10 mv with minimum signal input. 

Whether the noise limitation is set by the micro¬ 
phonics in the input tube or by the Johnson noise 
of the input circuit, the best signal-to-noise ratio is 
obtained if the input impedance of the amplifier is 
large as compared to the resistance of the ther¬ 
mistors. A high-input impedance was obtained by 
the use of a 100-megohm grid resistor and the largest 
possible cathode resistor in keeping with the gain 
requirement. The large cathode resistor provides 
sufficient bias to prevent the lowering of the input 
impedance due to grid current. 

Since high impedances are involved, it is neces¬ 
sary that all wires and components of the input 
circuit be very rigid and well insulated. The input 
impedance at the coupling unit is about 30 meg¬ 
ohms. 

The frequency response of the preamplifier is 3 db 
down at 1,000 and 2,000 c. In the investigation of 
the pulse distortion produced by the final amplifier 
it was convenient to work with a fairly wide-band 
amplifier. It would probably be desirable to narrow 
the bandwidth of the preamplifier and use a cathode- 
follower output circuit to match the line. 

Power Supply Unit. The voltage-regulated power 
supply for the preamplifier consists of a high- 
voltage regulator of the conventional degenerative 
type incorporating a pentode amplifier, and a fila¬ 
ment supply which is regulated by three 6L6 tubes. 
For the filament supply the voltage fluctuations are 
applied to the screens of the 6L6’s in such a manner 
as to oppose any change in the output voltage. The 
potentiometer to which the screen of a 12SJ7 is 
connected controls the amount of a-c applied to the 
screen for compensation and also varies the d-c 




336 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


screen bias. The output voltage is adjusted by 
means of the potentiometer which determines the 
potential of the control grid. 

Final Amplifier. The function of the final ampli¬ 
fier is to provide sufficient power for writing on elec¬ 
trolytic recording paper and also to present the elec¬ 
trical pulse in such a manner as to give the most 
intelligible record. The matter of providing suffi¬ 
cient power is straightforward while that of pre¬ 
senting the pulse to best advantage is far from 
simple. Considerable work, both experimental and 
theoretical, has been done to determine the optimum 
passband and phase shift of the final amplifier. The 
bolometer arrangement is such that a heat image 
which sweeps across the two bolometer strips pro¬ 
duces two electric pulses of opposite polarity. After 
the pulse passes through the preamplifier, the shape 
approximates a single sine wave, with duration 
approximately equal to the time for the heat image 
to traverse the bolometer strips. 

In order that such a pulse pass through an ampli¬ 
fier without distortion, theoretically the passband 
should extend from a frequency of zero to infinity. 
The amount of energy in any frequency interval 
may be computed, and for the SSD pulse, at a scan¬ 
ning rate of 6 degrees per second, the maximum 
energy lies near 9 c and a passband extending from 
0 to about 20 c would pass most of the energy. An 
amplifier with a large number of bandwidths was 
used to determine the band for which the signal-to- 
noise ratio was most favorable. A passband from 
6 to 25 c appears to be as satisfactory as any. 

Because of the phase-shift and frequency response 
characteristics of the amplifier, the pulse is distorted 
in its passage through the amplifier. The best re¬ 
sults in the field have been obtained with an output 
pulse which has one strong pulse in one direction 
and a reversed pulse on each side. The central peak 
is printed on the paper and the two smaller peaks, 
which are of opposite polarity, cancel random noise 
pulses which may occur. If the spaces between the 
traces made on successive scans are small enough 
so that the adjacent traces almost merge, the black 
signal indication has a white border on each side 
(Figure 33). 

In sector scanning the polarity of the pulse from 
the bolometer is different in the two directions of 
scan. The inversion is accomplished by a phase 
inverter tube in the final amplifier. A relay which 
is operated by contacts in the recorder switches from 


one side of the inverter to the other when the direc¬ 
tion of motion of the scanning head is reversed. 

The total gain of the amplifier is about 3,500. A 
calibrated attenuator permits gain adjustment in 
steps of 3 db, from 0 to 40 db. The switch on the 
input changes the gain by 40 db. Such an extreme 
range is not necessary in the field but is convenient 
for laboratory measurements. 

Representation Unit 

The SSD recorder was a rebuilt British AS/3-type 
chemical recorder designed for underwater sound 
work. A Pt-Ir stylus is used as the marking elec¬ 
trode and moves across the paper at a constant rate. 
The recording paper, which is damp, is fed from an 
airtight humidor and passes over a Monel roller 
which acts as the second electrode. In the earlier 
tests Sangamo fluorescent recording paper was used. 
With this paper a positive voltage on the stylus 
produces a black mark; a negative voltage has no 
effect. The impedance of this paper is about 10,000 
ohms and about 0.2 ma at 2 volts is required to 
produce a signal of medium intensity. 

In the later work Fax recording paper was used. 
The best results have been obtained by using the 
same electrode materials as were used with the 
Sangamo paper; however, for this paper the stylus 
should be negative. The paper has the advantage 
of making a black-on-white record, whereas the 
Sangamo paper makes a black-on-pink record. The 
impedance of the Fax paper is about 800 ohms and 
about 15 ma at 12 volts are required. 

The Fax recording paper has several disadvan¬ 
tages. The stylus wears quite rapidly and the roller 
must be cleaned after a few hours of operation. Air 
circulation within the recorder is critical; if the 
paper dries too quickly no marking is obtained and 
if it dries too slowly the record darkens almost 
immediately. The paper is easily contaminated and 
at present deteriorates while in storage. 

9104 Field Tests 

Cape Henlopen Tests, February 21 to 24, 1944 

The first official tests of the SSD equipment were 
conducted in cooperation with the Bureau of Ships 
at Fort Miles, near Lewes, Delaware, at night under 
rather favorable atmospheric conditions. The Surf 
Club, which served as headquarters, is located 50 
to 100 yards from the sea. To the east and southeast 







337 


THE STABILIZED SHIP DETECTOR [SSD] 


is open ocean; Cape May is about 12 miles north¬ 
east. All tests were run with targets against a sea 
horizon. 

On the night of February 21, Model 1 SSD was 
mounted on the observation platform of the Surf 
Club, roughly 50 feet above the sea. The only chance 
targets which were ranged by radar were a tanker 
and a cargo vessel. The SSD record showed that the 
tanker was detected by the SSD to a maximum 
range of 14,100 yards. The cargo vessel Admiral 
Byrd was followed to about 14,700 yards. 

For the tests of February 23 and 24, an LCT and 
an LCI were used as controlled targets for Model 2 
SSD (64-millimeter diameter optics) mounted 35 
feet above the sea. 

The ranges at which targets may be detected 
depend considerably upon the aspect of the target 
vessel; hence for each test two ranges are given. 
By the minimum range (min R ) is meant the clos¬ 
est approach of the target vessel without detection 
and would be expected to correspond to a bow or 
stern view (aspect 0 or 180 degrees) for the type of 
vessels used. The maximum ranges (max R) are 
simply the greatest distances at which the targets 
were actually detected. A summary of the range 
results in yards is given in Table 6. 


Table 6. Range data for tests made February 23 and 
24, 1944. 


Date 

Target 

Min R 

Aspect 

Max R 

Aspect 

Feb. 23 

LCI 

9,200 

10° 

14,100 

125° 

Feb.23 

LCT 

8.700 

0° 

10,500 

150° 

Feb. 24 

LCI 

10.600 

25° 

15,100 

130° 

Feb.24 

LCT 

7,300 

0°(?) 

15,100 



These tests showed that the minimum range of 
approach without detection of an LCI (bow or 
stern view) was about 9,000 yards and of an LCT 
(bow or stern) about 7,300 yards. With more 
favorable ship aspects the LCI was reliably de¬ 
tected up to 15,000 yards and the LCT up to 10,500 
yards. In one special test in which the LCI maneu¬ 
vered without lights in a complex pattern it was 
continuously detected at all ranges within 14,000 
yards, except for one brief period at 10,500 to 12,000 
yards range with bow aspect. 

A portion of the SSD record made on February 24 
is reproduced in Figure 32. The trace of the LCI is 
marked with an X. The signal due to the lightship 
at 7,100 yards is indicated on the record. The bear¬ 


ing accuracy of the type of radar used for ranging 
was not sufficiently great to check the accuracy of 
the bearings read from the SSD record with a pre¬ 
cision of 0.25 degree. Occasionally information from 
the SSD record assisted radar in selecting the proper 
target. 

The radiation temperatures of various parts of 
the LCT and LCI were measured by means of a 
radiation pyrometer. 19 The only parts of the LCT 
which were warmer than the air (about 5 C) were 
the galley stovepipe, which was 8 inches by 5 feet, 
and the pipe on the bilge pump, which was not well 
exposed to horizontal view and was at 31 C. No 
conspicuous warm areas on the LCI could be found. 
It was concluded that the ship was a thermal target 
by virtue of its silhouette against the background. 

The visibility was good on both February 23 and 
24 although the first night was slightly clearer. The 
visibility was not estimated and the relative humid¬ 
ity was not determined. 

Following those excellent tests, certain features 
requested by the Bureau of Ships were incorporated 
in the equipment better to adapt it for operational 
use. These included weatherproofing, ambient tem¬ 
perature compensation, a regulated power supply 
unit for the a-c preamplifier operated directly from 
the ship’s power lines, and the determination of the 
best amplifier pass band for the detection of weak 
signals in a noise background. 

Operational tests of the SSD aboard the USS 
Marnell on May 8, 1944, indicated that the above 
improvements had been satisfactorily accomplished 
but indicated the need for a reduction in micro¬ 
phonics, a correction for the yawing of the ship to 
permit the identification of a weak signal on the 
chart in a rough sea, and a reduction in the vibra¬ 
tion of the stable table furnished by the Navy. 

Work was undertaken to reduce the amplitude of 
vibration of the stable table. The microphonics, 
which were found to be caused essentially by the 
gas swish in the bolometer cell, were effectively 
eliminated by modifying the cell so that the bolom¬ 
eter was enclosed in a very small housing. An 
auxiliary unit for yaw correction was constructed. 
This unit contains means for adjusting the SSD 
recorder to give true bearing and indicates to the 
operator by means of three dials at all times the 
relative bearing of the scanning head, the true bear¬ 
ing, and the ship’s course. In addition, the driving 
mechanism for the scanning head and the power 









338 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 








THE STABILIZED SHIP DETECTOR [SSD] 


339 


supplies for the final amplifier and for the bolom¬ 
eter were improved. 

In tests on the Marnell, September 11 to 13, 
1944, a tanker with stern aspect was detected up 
to a maximum range of 6,000 to 7,000 yards under 
the unfavorable atmospheric conditions of 77 F air 
temperature, 90 per cent relative humidity, and low 
visibility. 

Marnell Tests , 20 October 23 to November 3, 1944 

The SSD equipment was again placed on the 
Marnell at Cape May, New Jersey, on October 22, 
1944, and remained on board until November 3. In 
the period between the departure from Cape May 
on October 23 and the arrival at New London, 
October 27, data were taken four nights outside 
New York Harbor; passing and harbor traffic pro¬ 
vided chance targets. Tests on a controlled destroyer 
escort target were run in the New London area on 
October 30 and 31 and November 1, as a demon¬ 
stration to Armed Services representatives. 

Scanning heads were mounted on two stabilized 
tables which were located on the top deck of the 
Marnell. The height above the water was about 
23 feet. The remainder of the equipment, which con¬ 
sisted of the recorder, amplifier, power supply, and 
mechanical driving unit, was set up in the experi¬ 
mental station on the deck below. Range informa¬ 
tion was obtained from the ship’s type SF-1 radar. 

The scanning motion of the head was at the rate 
of 6 degrees per second, and the full width of the 
record corresponded to a scanned sector of about 
45 degrees. A bolometer cell having a 64-millimeter 
aperture was used in these tests. 

If the chance targets, identified as freighters, 
Liberty ships, and merchants, are treated as a 
group the average definite signal range (defined as 
the distance at which the presence of a target is 
established under search conditions) is 9,950 yards. 
For these larger targets the maximum definite signal 
range was 18,000 yards, and the minimum 5,200 
yards. Very nearly 75 per cent of the ranges were 
9,000 yards or greater. 

The section of record reproduced in Figure 33 
was obtained between 04:15 and 06:00, October 24, 
when a convoy and escort vessels were encountered 
about 10 miles off Ambrose Lightship. This was the 
only SSD record obtained of a convoy at sea and 
indicates the resolution which is obtainable. 

For the tests off New London, the Breeman 


(DE-104) served as the target. 0 Maneuvers were 
planned to give bow, bow-quarter, broadside, stern- 
quarter, and stern aspects of the target vessel. The 
results obtained on the three nights were reasonably 
consistent; the average definite signal ranges for the 
runs on the three nights were 6,850, 6,770, and 6,010 
yards. 

A summary of the data in terms of the aspect of 
the target vessel is given in Table 7, in which the 
tracking signal range is defined as the maximum 
distance at which the bearing can be obtained for 
a target, the presence of which has previously been 
verified. 

Table 7. Target Breeman-, October 30 to November 

1, 1944. 


Definite Signal Tracking Signal 
Aspect Runs Range Range 




Avg 

Max 

Min 

Max 

Min 

Broadside 

5 

5.880 

8.000 

4,700 

9.500 

5,400 

Stern 

11 

7.045 

10.000 

5,100 

11.000 

6.100 

Stern quarter 

6 

7,350 

10.800 

5.000 

11.200 

5,500 

Bow quarter 

3 

5,270 

6,000 

4,400 

6.900 

5.800 

Bow 

3 

3.970 

4,500 

3,400 

6,700 

4.000 

All aspects 

28 

6.380 






Cape Henlopen Tests, March 20 to 24, 1945 

In order to increase the receiver responsivity, 
bolometer cells 100 and 150 millimeters in diameter 
were developed and constructed in cooperation with 
the Bureau of Ships. The mounting for the gas- 
cooled thermistor bolometer was modified to reduce 
the volume of gas surrounding the bolometer and 
hence the microphonics due to gas swish. One of 
the SSD amplifiers was modified to provide the 
greater power required by Fax chemical recording 
paper, which was found to be superior to both the 
Sangamo fluorescent paper previously employed 
and to Teledeltos paper. 

The SSD equipment previously field-tested was 
designed for sector scanning (50 degrees or less). 
In order to permit the visualized operational need 
for means of changing quickly from sector scanning 
to 360 degrees search or vice versa, a design was 
completed which permits unattended 360-degree 
search and which can be adjusted to permit either 
or both scanning heads to be converted quickly to 

c Radiation temperature measurements of exposed sur¬ 
faces of the target vessel and of the horizon background 
were made during the test at sea on October 31, 1944. 20a 










340 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


Figure 33. 


SSD record made of a convoy near Ambrose Lightship. 












341 


THE STABILIZED SHIP DETECTOR [SSD] 




















342 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


sector scanning. An improved, more powerful servo¬ 
mechanism required for the new recorder con¬ 
structed for the dual operation of scanning heads 
was designed. 

The first three nights of testing yielded little data 
of interest, due to light traffic and radar difficulties; 
however, continuous records were made. During the 
night of March 23, there were many targets and the 
radar (type SG-1) was in operation. The scanning 
head was mounted 15 feet above sea level and at 
this height ships, probably freighters, were detected 
and tracked up to ranges of 20,000 yards, even 
though at 18,000 yards the ships were 20 feet hull 
down. During these tests the temperature was 55 F 
and the relative humidity was 54 per cent (Figure 
34). It will be noted from Figure 34 that a part of 
the record was made with a cover placed over the 
window. The signals written under this condition 
constitute the electrical noise background of the 
instrument. It will be seen that with the cover 
removed the general noise background is consid¬ 
erably increased; the increase is perhaps about 6 db. 
Hence, for these test conditions the thermal back¬ 
ground set the limit of operation, and consequently 
greater sensitivity would not have given greater 
detection ranges. The effect of the thermal back¬ 
ground might be reduced by decreasing the field of 
view, but for shipboard operation this is not prac¬ 
ticable. Under these conditions the scanning speed 
might be increased without decreasing the threshold 
range. 

Later during the same night with the SSD 
mounted 40 feet above sea level (distance to 
horizon 14,000 yards) and at an ambient tempera¬ 
ture of 55 F and relative humidity of 74 to 80 per 
cent, passing ships, probably freighters and coast¬ 
wise traffic, were detected and tracked up to ranges 
of about 23,000 yards. The section of the record 
shown in Figure 35 shows three vessels passing the 
mouth of Delaware Bay; the radar did not resolve 
the targets. In this record a surprisingly large signal 
is given by a can buoy at 3,250 yards. 

Tests aboard the Marnell and at Cape Henlopen, 
June 5 to 19, 1945 

The final experimental model of the SSD equip¬ 
ment having an improved scanning head with a two- 
speed follow-up system was designed and con¬ 
structed. This new head contains a much more rigid 
mounting for use on the stabilized platform, and a 


new follow-up system consisting of a servo motor 
geared to the rotating part of the head and to con¬ 
trol transformers, operated at 1 and 36 speed. For 
this scanner, the drive unit, built by the Ford 
Instrument Company for the Bureau of Ships, con¬ 
sisted of a synchronous motor geared to 1- and 36- 
speed synchro differential generators. The course of 
the ship on which the test was made was put into 
the scanning motion electrically. The drive unit was 
linked mechanically with the recorder. 

A second such scanning head, the mechanical 
drive of which was constructed by Bendix Marine 
to Harvard specifications under a Bureau of Ships 
contract, was also available. 

The final experimental model of the SSD equip¬ 
ment was installed on the Marnell on June 5, 1945, 
at New York City. On the nights of June 6-7 and 
7-8 the stable tables drifted, which necessitated 
frequent leveling of the scanning head in order to 
detect targets. Several targets were encountered on 
the night of June 7-8 and operational ranges were 
determined for a number of ships of unknown types 
under conditions of ambient temperatures of 55 de¬ 
grees to 60 degrees, 2-centimeter precipitable water 
vapor content. The operating ranges varied from 
6,300 to 14,000 yards. 

On the night of June 9 an unidentified vessel was 
followed from New York to New London. Since the 
ship moved slowly it was possible to open and 
close range at will, thus making possible the deter¬ 
mination of several threshold ranges. A portion of 
the SSD record is shown in Figure 36. During the 
6-hour period, the bearing of the target was shown 
on the record when the range was not greater than 
14,000 yards, except during a brief interval when 
the scanning rate was doubled (12 degrees per sec¬ 
ond) ; the threshold was then reduced to about 
11,000 yards. From the ship’s log the visibility was 
given as 2 miles for 5 hours of the run. 

Extensive tests were made with a controlled DE 
target on the night of June 11-12 and on a con¬ 
trolled DD target on the night of June 12-13. There 
was apparently little difference between the atmos¬ 
pheric conditions on these two nights. The average 
detection ranges for the DE and DD were 8,030 
and 8,250 yards, respectively, which indicate that 
the two vessels were approximately equivalent heat 
sources. 

The data are analyzed in Table 8. It will be noted 
that for all aspects the average signal range was 




THE STABILIZED SHIP DETECTOR [SSDJ 


343 


OVERFALLS 

LIGHTSHIP 


'CAN BUOY 
3250 YD 



RADAR DID NOT 
RESOLVE TARGETS 


23100 YD 


23000 


22800 


22600 

22500 

22300 


TARGETS 



Figure 35. SSD record made at mouth of Delaware Bay. 























FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


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Figure 36. SSD record made aboard the Marnell near Cape Henlopen. 











































EXPLORATORY EQUIPMENT USING THE LEAD SULFIDE CELL 


345 


greater in these tests than in the October 1944 
tests; also, the minimum signal range was appre¬ 
ciably greater in all cases except for bow aspect. 
During the October tests the maximum sector 
scanned was about 40 degrees or less, in order to 
scan the target more frequently. During the June 
tests the sector which was scanned was 60 to 65 
degrees. 

Table 8. Data taken aboard the Marnell and at Cape 

Henlopen, June 5 to 19, 1945. 


Definite Signal Tracking Signal 
Range Range 


Aspect Runs 

Avg 

Max 

Min 

Max 

Min 

Broadside 

6 

10,340 

12.800 

8.800 

12.080 

10,200 

Stern 

7 

7,890 

9.150 

6,800 

10,800 

7,540 

Stern quarter 

5 

8,930 

9,700 

8.000 

10,900 

9,600 

Bow quarter 

3 

8.520 

9,400 

7,600 

13,000 

9,100 

Bow 

4 

4,475 

6,300 

3,600 

6,300 

3,600 

All aspects 

24 

8,100 






Atmospheric conditions during these June tests 
were less favorable than during the October tests, 
due to considerable haze which existed during all 
of the test period. The average temperature was 
60 F and from the ship’s log the visibility was 
5 miles on the night of June 11-12 and 5 to 10 miles 
on June 12-13. The water content of the atmos¬ 
phere was 2.4 centimeters per sea mile. The sea was 
rougher during the June tests; the maximum values 
of roll and pitch recorded on the night of June 11-12 
were 12 and 7 degrees, respectively. The Marnell 
yaws considerably under these conditions. 

The greater ranges obtained during these tests 
may be attributed to the use of a cell having a 
larger aperture, to improvements in recording, and 
to more accurate correction for yaw and course. 

A section of the record obtained during the night 
of June 13 is reproduced in Figure 37. This is one 
of the better records secured during the controlled 
target tests and shows the greatest bow range 
which was obtained. It is of interest to compare 
signal strengths at equal ranges for different aspects. 
It will be seen that the “bow signal” is definitely 
weaker than the “stern signal;” the relatively short 
ranges for bow aspect result from the combination 
of weak signals and the rapid closing of range. 
During two bow runs the range was closed at 
approximately 1,000 yards per minute. 

In order to check the overall sensitivity of the 


equipment, the apparatus which was used in the 
March 1945 tests at Cape Henlopen was set up at 
the Bureau of Ships Test Station (Cape Henlopen) 
for comparison with that used on the Marnell in 
these June tests. There was no appreciable differ¬ 
ence in ranges obtained with the two sets of appa¬ 
ratus. Typical ranges observed at the Test Station 
at an ambient temperature of 71 to 81 F for several 
different types of targets were as follows: U. S. cargo 
ships, 8,000 to 12,000 yards; pilot boats, 8,300 to 
8,800 yards; tug, 8,900 yards; U. S. tankers, 7,600 to 
11,000 yards; British tanker, 13,000 yards (the 
variation in range is due, in part, to varied aspects 
of vessel viewed). These ranges compare favorably 
with those which were obtained on shipboard. The 
haze conditions at Henlopen were similar to those 
encountered in the New York and New London 
areas. The precipitable water varied between 3.2 
and 4.0 centimeters per sea mile at Cape Henlopen 
as compared to 2.0 to 2.4 centimeters per sea mile 
on board ship. 

During the March test at Cape Henlopen, when 
20,000-yard ranges were obtained, there was little 
or no haze and about 1.4 centimeters of water per 
sea mile. 

9105 Present Status 

The stabilized ship detector was adopted by the 
Navy for operational use. Several production con¬ 
tracts were let for the manufacture of models of the 
SSD equipment. At the end of the war preproduc¬ 
tion models of the different components of the SSD 
had been produced and were being given tests prior 
to the final design of the components for quantity 
production. 

911 EXPLORATORY EQUIPMENT USING 
THE LEAD SULFIDE CELL FOR 
MILITARY PURPOSES 

911,1 Introduction 

The University of Michigan, under Contract 
NDCrc-185, was requested by Section 16.4, NDRC, 
to construct and test a preliminary device utilizing 
a lead sulfide [PbS] photoconductive cell as the 
sensitive element, for the detection of low-tempera¬ 
ture military targets by means of their self-emitted 
thermal radiation. 








346 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 





Figure 37. SSD record made aboard the Marnell near Cape Henlopen. 




















EXPLORATORY EQUIPMENT USING THE LEAD SULFIDE CELL 


347 


This section of Chapter 9 describes briefly an 
exploratory receiving apparatus using this type of 
cell, and the detection tests carried out with this 
equipment using men and ships as targets. Since the 
PbS cell is most sensitive in the intermediate infra¬ 
red region, this equipment has been designated as the 
IIR (intermediate infrared) receiver.- 1 

This exploratory test equipment consists of an 
oscillating mirror, 15 centimeters in diameter, which 
sweeps the minute field of view of the receiver 
through an angle of 0.015 radian about 55 times per 
second; the detector cell, its sensitive surface located 
in the focal plane of the mirror and cooled with 
solid CO 2 ; a battery-operated amplifier with which 
to amplify the electric signal produced when the 
image of a thermal discontinuity traverses the sen¬ 
sitive element, including a narrow passband cir¬ 
cuit tuned to twice the frequency of the oscillating 
mirror to increase the signal-to-noise ratio; and a 
signal-output-indicator meter. 

General Description 

The apparatus which was designed consists in 
principle of an oscillating mirror with the sensitive 
surface of the PbS cell placed at the focal point. 
If the object subtends a sufficiently small angle, the 
apparatus is so aimed that the cell receives radia¬ 
tion from the background with the mirror in one 
extreme position, from the object when the mirror 
is at the center, and from the background again 
when the mirror reaches the other extreme position. 
The “scanning frequency” is about 55 c. This is 
twice the 27.5-c frequency of oscillation of the 
mirror, since the image of the source is swept across 
the sensitive element from left to right and also 
from right to left once in each complete oscillation 
of the mirror. When the amount of radiation re¬ 
ceived from the object is greater or less than that 
from the background, the resistance of the PbS cell 
is different while the mirror field of view includes 
the object than when the field of view includes 
only an otherwise uniform background. This results 
in a periodic electric impulse which, after amplifi¬ 
cation, operates the signal-output-indicator meter. 
A photograph of the apparatus is shown in Fig¬ 
ure 38. 

For radiation having wavelengths from 2.5 to 
3.5 p, this* receiver was found to be capable of 
detecting a radiation flux density of about 10' 12 


watt per square centimeter, corresponding to about 
10‘ 10 watt incident on the sensitive element when 
cooled with solid C0 2 . For a black-body source at 
temperatures near 50 centigrade degrees the total 
flux density necessary to provide this amount of 
power in the 2.5- to 3.5-p band is about 400 times 
larger than the figure given above. This takes into 
account the total power radiated by such a source 
over all wavelengths, to much of which the lead 
sulfide detector is insensitive. 



. .. ... i 

Figure 38. Photograph of intermediate infrared de¬ 
tector. 

911,3 Description of the Component Parts 

Optical System 

The oscillating mirror and the driving motor are 
supported in an aluminum casting of special design. 
The precision quality, parabolic mirror has a diam¬ 
eter of 15 centimeters, a focal length of 6 centi¬ 
meters, a circle of confusion about % 4 inch in diam¬ 
eter, and is gold-plated to provide high infrared 
reflectivity. The sensitive surface of the PbS cell is 
brought to the exact focal point of the mirror by 
adjustment of the four screws on which the base 
plate of the cell and shield assembly is mounted. 





348 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


The field of view of the receiver was determined 
by placing a flashlight bulb at a distance of 20 feet 
in front of the receiver and plotting the output 
signal in decibels against the angular coordinates 
of the “point source.” The “effective field of view,” 
defined as the region enclosed by the —6 db iso¬ 
decibel line, when measured with reference to the 
maximum decibel reading obtained, has evidently 
one value when the mirror is stationary and another 
when it is oscillating. 

With the mirror stationary, the point source was 
modulated by operation from an a-c power supply. 
The effective field of view determined experimen¬ 
tally by this method was approximately rectangular 
in shape, 0.0025 radian wide by 0.0125 radian high, 
and is hereafter called “the mirror field.” 

When the mirror oscillated, the scanning of the 
mirror field across the point source modulated the 
radiation incident on the detector. The height of 
the effective field of view was found to be the same 
as with the mirror stationary, but as expected, the 
width of the field was found to depend upon the 
scanning angle, which may be varied. This effective 
field of view is hereafter referred as to the “oscil¬ 
lating field.” All the final tests were made with a 
scanning angle of 0.015 radian, which was found to 
produce an oscillating field width of 0.007 radian. 

Scanning Mechanism 

An adjustable cam, visible on the end of the 
motor shaft, drives the mirror through any desired 
scanning angle. The motor used is an 8-volt, 1/200- 
hp, d-c motor the speed of which on 6 volts is 
about 1,650 rpm, causing the mirror to oscillate 
about 27.5 times per second. 

The optimum adjustment of the scanning angle 
so as to obtain the maximum output indication 
depends upon the angular size of the object which 
is to be detected. The relative time spent “on” and 
“off” the object on each side of the scan affects the 
wave shape of the signal and therefore the magni¬ 
tude of its fundamental frequency component. The 
magnitude of the indication is proportional to the 
amplitude of only the fundamental component, 
which is in turn dependent upon the wave shape of 
the signal from the cell. 

To produce a signal with a fundamental frequency 
of twice the oscillating frequency of the mirror, the 
instantaneous mirror field must scan through an 
angle equal to or greater than the sum of the angular 


size of the mirror field added to the angular size of 
the target. The maximum amplitude of this signal 
from a target occurs when the scanning angle is 
adjusted so that the time spent “off” the target on 
each side approximately equals the time spent in 
scanning across the target. 

The Detecting Element and the Amplifiers 

The lead sulfide photoconductive cell (described 
in Chapter 3) has been developed in this coun¬ 
try by Northwestern University under Contract 
OEMsr-235. 22 The effect of background illumina¬ 
tion on the response of the cell to a modulated signal 
is much smaller than for the thalofide [TF] cell. 
Typical cells made to date have a peak response 
near 2.5 p and a long-wavelength threshold near 
3.5 p. A spectral response curve for one cell is shown 
in Figure 39. 



1.2 1.4 1.6 IJB 2D 22 2.4 2.6 2£ 3.0 3.2 3.4 3.6 3.8 


WAVELENGTH IN MICRONS 

Figure 39. Spectral-response curve for photoconduc¬ 
tive cell. 

The responsivity of the PbS cell increases with a 
decrease in its temperature. Although the effect of 
cooling may vary from cell to cell, one cell showed 
a gain in signal-to-noise ratio of 25 db at the tem¬ 
perature of solid C0 2 as compared with room tem¬ 
perature. 

Over a wide range the response of the cell is essen¬ 
tially independent of frequency; most cells show a 
practically flat response at room temperature to fre¬ 
quencies of from 30 to 1,080 c. When these cells are 
cooled the change of cell response with frequency is 
somewhat greater than at room temperature, but is 
still not so large as for TF cells; at 1,080 c the 
response from three PbS cells, cooled with solid C0 2 , 
was down by 6, 7, and 8 db, respectively, as com- 



































EXPLORATORY EQUIPMENT USING THE LEAD SULFIDE CELL 


349 


pared with the response voltage at a signal modula¬ 
tion frequency of 90 c. 

The cell used in the receiving equipment de¬ 
scribed herein was designed for cooling with solid 
C0 2 . It had a sensitive surface about 0.3 by 1.5 
millimeters. At room temperature its signal-to-noise 
ratio w T as 67 db for an incident flux of 1 micro- 
hololumen from a tungsten source operated at or 
near a temperature of 2848 degrees K, when the 
load resistor was 1 megohm and the applied volt¬ 
age 22.5 volts. Its response at room temperature w T as 
2.5 db less at 1,080 c than at 90 c. 

The PbS cell is connected in series with a 
5-megohm load resistance and a 45-volt battery so 
that any change in the resistance of the cell results 
in a corresponding change in voltage across the 
fixed load resistor. The fundamental frequency of 
this output voltage is the same as the scanning 
frequency (i.e., twice the mirror oscillation fre¬ 
quency). The wave shape of this voltage will de¬ 
pend on the relation of the angle of scan to the 
angular size of the object and to the angle of view 
of the mirror. This signal is transmitted to the 
amplifier. 

The amplifier for the HR receiver consists of 
three units: a preamplifier, a main amplifier, and a 
tuning section which plugs into the main amplifier. 
The preamplifier consists of a resistance-coupled 
amplifier, with self-contained batteries for plate and 
cell supply. A wire-wound load resistor for the cell 
was used in order to eliminate noise caused by the 
cell current passing through the load resistor. The 
remaining noise originates principally or entirely 
in the cell. The preamplifier is hung in a box be¬ 
neath the casting to permit the shortest possible 
leads from the cell to the grid of the first tube. 
Antivibration mounts isolate the amplifier mechan¬ 
ically to avoid microphonics which otherwise might 
be caused by vibrations from the oscillating 
mirror. 23 

The main amplifier has been completely de¬ 
scribed in a contractor’s report. It is entirely 
battery-operated, and consists of an audio-frequency 
amplifier with calibrated attenuators and an output 
decibel meter of the slow speed, rectifier type. 

The amplifier tuning unit is a General Radio 
Company’s type 760-A sound analyzer. It has a 
bandwidth, at the half-power points, of 2 per cent 
of the frequency to which it is tuned. This sharply 
tuned unit is used because the noise voltage appear¬ 


ing at the output meter is proportional to the square 
root of the bandwidth. Thus by tuning the narrow 
band-pass filter to the fundamental frequency of the 
signal, although the original wave shape of the 
signal is not maintained, it is possible to realize an 
appreciable gain in the signal-to-noise ratio. 

The total possible amplification from cell to out¬ 
put meter is approximately 150 db. The gain neces¬ 
sary to reach the noise level of the PbS cell is 
approximately 100 db. The signal is read on a meter 
which is used as an indicator. 

9,11,4 Threshold Sensitivity for a 
Low-Temperature Source 

The spectral sensitivity curve for one PbS cell 
(not the cell in the HR) is shown in Figure 39. 
Also shown are the spectral emission curve of a 
black body at 30 centigrade degrees and the prod¬ 
uct curve, on an arbitrary scale, of the 30 centi¬ 
grade degrees black-body emission curve and the 
cell spectral response curve. The product curve 
shows that about 95 per cent of the response of this 
cell to radiation for a 30-eentigrade degree source 
is due to only that part of the entire radiation 
which has a wavelength of from 2.5 to 3.5 p. 

Threshold flux densities which the receiver can 
detect, based on a signal-to-noise ratio equal to one, 
have been determined and are given in Table 9. The 
values given do not take into account diminution 
by atmospheric absorption but represent the energy 
densities at the receiver necessary to produce a 
signal equal to the noise. 

Table 9. Threshold signal flux density for PbS 
receiver (black-body radiation distribution assumed 
from source at T centigrade degrees against uniform 
background at 25.5 C). 


T 

(degrees C) 

Microwatts/cm 2 
(2.5- to 3.5-n 
region) 

Total 

radiation 

30 

1.02 X 10- 6 

6.9 X 10- 4 

40 

1.02 X 10- 6 

5.3 X 10- 4 

50 

0.95 X 10- 6 

3.4 X 10- 4 

60 

0.80 X 10- 6 

2.6 X 10" 4 


9115 Field Tests 

Personnel Detection 

The purpose of the first test was to determine the 
ability of the IIR receiver to detect radiation from 
a man at night against a sky background and 










350 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


against a foliage background for comparison with 
the reported sensitivity of the PND. 10 The tests 
were conducted near Ann Arbor, Michigan, on a 
hazy night with stars and moonlit sky; relative 
humidity was 94 per cent, temperature 59 F. A 
clothed man at 1,000 feet silhouetted against the 
sky produced a signal 15 db above the noise. The 
same man at 900 feet against a foliage background 
produced a signal 18 db above the noise. These were 
not threshold ranges and, therefore, indicate per¬ 
sonnel detection ranges of the same order as those 
reported for the PND, with which a man can be 
detected against a foliage background at a maxi¬ 
mum distance of 1,500 feet. 

Ship Detection 

A second field test was conducted to obtain ex¬ 
perience on the detection of ships and to obtain 
some idea of the operation of the apparatus through 
several miles of atmospheric attenuation. This test 
was made at the Grosse Pointe (Michigan) Yacht 
Club, overlooking Lake St. Clair, on the night of 
October 2, 1945. The receiving equipment was 
located about 25 feet above the water level and 
freighters were observed as they passed up or down 
the ship channel. Bearings of the ships were taken 
with a transit at the time the ships were detected, 
and from a large scale map of the channel the 
approximate ranges were determined. The most dis¬ 
tant target observed was a lighted freighter on the 
horizon about 8 miles away. It produced a signal 
24 db above the noise. The lights on this ship were 
barely visible to the eye at this range, but it is 
believed that a large part of the detected signal 
came from these lights. Since it was not feasible to 
extinguish the lights and since for the 2.5- to 3.5-p 
region no other means were available for isolating 
their radiation from that of the ship, a measure of 
the ability of the detector to respond only to the 
thermal radiation of the ship itself was not obtained. 

The purpose of the third field test was to deter¬ 
mine an order of magnitude for the detection range 
of a ship without lights. This test was conducted on 
the night of October 22, 1945, at the Bureau of 
Ships Test Station, Cape Henlopen, Delaware. An 
SSD 18 was operated simultaneously with the HR 
receiver so that proper comparison could be made 
between the two, using the same target and target 
distance and under the same weather conditions. 
The Callao, a 1,000-ton steam-driven ship, 185 feet 


long and 30 feet wide, served as target. Its stack is 
about 5 feet in diameter, reputedly quite hot, and 
is equally visible when observing the port, stern, 
or starboard aspect of the ship. During this test 
there was a very light haze and a bright moon. The 
temperature was 64 F and the relative humidity 
94 per cent. The Callao was detected at a maximum 
range of 4,400 yards with stern aspect. The corre¬ 
sponding threshold range for detection by the SSD 
was between 8,100 and 9,000 yards, or about twice 
that of the HR receiver. 

911-6 Evaluation of Results and Proposals 
for Future Work 

The results of the Cape Henlopen test provide 
the only reliable data obtained on the performance 
of the IIR receiver as a ship detector. 

The SSD is essentially a nonselective receiver 
which responds almost uniformly to equal amounts 
of radiant energy at any wavelength from the visible 
out to about 15 p, the long wavelength transmission 
limit of the rock-salt window which covers the 
bolometer element. The maximum radiation from a 
low-temperature source such as a ship occurs near 
9 p and fortunately coincides with the well-known 
atmospheric transmission “window” from 8 to 13 p. 
As a result, the greater part of the radiation to 
which the SSD responds lies within the “window” 
region of 8 to 13 p. 

The IIR receiver, on the other hand, is sensitive 
only to wavelengths no longer than about 3.5 p. 
Only about 0.1 per cent of the total energy radiated 
by the ship is of shorter wavelength than 3.5 p. 
Moreover, a strong water vapor absorption band 
overlaps the region from 2.5 to 2.8 p and a weaker 
band extends from 2.8 to approximately 3.3 p. 
Within the regions from 2.5 to 3.5 p lies most of the 
radiation from the ship to which this receiver 
responds. 

One may therefore say that, although the thresh¬ 
old sensitivity of the two receivers to black-body 
radition from a low-temperature source is about the 
same, the transmission characteristics of the atmos¬ 
phere alter the spectral quality of the radiation 
from a low-temperature source, during its passage 
to a distant receiver, in such a way as to dis¬ 
criminate against the IIR receiver in comparison 
to the SSD. The high humidity at the time of the 
tests accentuated this discrimination. 



THE INFRARED RANGEFINDER 


Present Status 

The promising results of the preliminary tests 
made with this exploratory apparatus appear to 
warrant further development of the PbS cell for 
use as a high-speed intermediate infrared detector. 
Moreover, the cooled-type PbS cell already devel¬ 
oped under Contract OEMsr-235 apparently has 
sufficiently desirable properties to justify the fur¬ 
ther development of HR receivers utilizing this 
type of heat-sensitive element for personnel detec¬ 
tion, ship detection, or other military applications. 
Perhaps some other type of photoconductive cell 
could be found in which the sensitivity might extend 
out to about 4.5 p. 

AVork has been initiated at Northwestern Univer¬ 
sity under Contract OEMsr-235 in an attempt to 
develop cells (utilizing PbS and other materials) 
that are sensitive beyond 3.5 p, in order to make 
use of the narrow atmospheric “window” in the 
region of 3.4 to 4.4 p. On the basis of the test results 
described above, it seems likely that if such a cell is 
successfully developed, a receiver could be built 
having a greater range and a more rapid response 
than the other infrared detecting equipment now 
in use. 

9 12 THE INFRARED RANGEFINDER 
Introduction 

Considerable interest existed within the Navy 
concerning the possibility of developing a far infra¬ 
red (8 to 14 p) device which, in addition to detect¬ 
ing a target by bolometric response to temperature 
differences between the target and its background, 
could also fix the distance of the target from the 
detector. This interest in an infrared rangefinder 
which could be used 'at night without detection by 
the enemy gave impetus to the establishment at 
Harvard University of a project to develop such a 
device. Contract OEMsr-60 was set up under 
Project Control NO-183 by a request from the 
Bureau of Ordnance. 

The purpose of the project was the development 
of a rangefinder employing long-wave, thermal-type 
infrared radiation. 

The equipment developed determines at night the 
range of a ship to 10 per cent or less and its bearing 
to 1 minute of arc or less. The equipment is effec¬ 
tive to at least 5,000 yards. 


351 

Automatic horizontal guiding was a necessary 
auxiliary development to make the equipment use¬ 
ful on an invisible target. Up to 5,000 yards range, 
the equipment will automatically follow a target in 
the horizontal plane to less than 1 minute of arc. 

Possibilities for improvement in the equipment 
are the following: 

1. Increase in sensitivity. 

2. Increase in accuracy of ranging. 

3. Automatic vertical guiding and self-stabiliza¬ 
tion. 

General Description 

A simplified schematic diagram of the infrared 
rangefinder 24 is shown in Figure 40. This instrument 
operates on the principle of the standard optical 
rangefinder which determines the range R from the 
interocular distance and the parallax. 

Heat radiations from a target ship T are focused 
on a sensitive bolometer at B. In this section, a 
bolometer and its associated optics are referred to 
as an eye. The electric signal generated at B by 
the heat radiations operates an automatic device 
which rotates the whole rangefinder about the 
point C and keeps the eye B always pointed at the 
target. Consequently, the other eye A is also pointed 
in the general direction of the target. However, if 
the eyes are parallel, it is necessary to rotate the 
eye A farther toward the target by the amount of 
the parallax angle. This further rotation of the eye 
is manually controlled by an operator so that both 
eyes are kept pointed always at the target. 

In the case of the standard optical rangefinder 
the operator performs a similar rotation of one eye 
with respect to the other by the amount of the 
parallax angle. He judges when both eyes are cor¬ 
rectly pointed by visual comparison of the images 
which they form. He reads the range from a dial 
which is moved in accordance with the parallax 
angle rotation. In the case of the infrared range¬ 
finder the electric signal received at B is compared 
electronically with that received at A, and the result 
registers on a meter. The operator judges when both 
eyes are correctly pointed by observation of this 
meter. The parallax angle is read from a dial which 
moves in accordance with the parallax angle rota¬ 
tion of the eye A. The parallax angle is related to 
the range R and the interocular distance Z by the 
expression P — (Z/R), p being the parallax angle. 






352 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 




Figure 40. Schematic diagram of infrared rangefinder. 


The detectors at points A and B are rotating 
squirrel-cage type nickel strip bolometers. This 
instrument consists of a number of very thin, 
blacked nickel strips spaced evenly around the 
surface of an imaginary cylinder, the strips being 
parallel to the vertical axis of the cylinder about 
which the strips rotate with a constant speed v. 

Automatic Following 

The schematic diagram, Figure 40, illustrates how 
bolometer B keeps the rangefinder pointed at the 
target. When the lens is pointed directly at the 
target, radiations arrive along its optical axis M, 
and an image of the target is focused at point 1. As 
each nickel strip sweeps through this target image, 
it is heated by the radiation. The strips are con¬ 
nected so that the successive heating of each strip 
produces a small alternating voltage in the associ¬ 
ated circuits. If the lens is not pointed directly at 
the target, the radiations will arrive along some 
other path, such as N. In this case, the image will 
be formed by the lens at the point 2. The angle 
between the radiation path N and the optical axis 
M is called the guide angle ft. 


The bolometer measures this small displacement 
ft of the image in the following manner. Each strip 
arrives at point 2 somewhat later than it arrives at 
point 1. Consequently, the alternating voltage wave 
produced by an image at 2 will be shifted in time 
relative to a hypothetical wave produced by an 
image at 1. This time shift is a measure of the 
image displacement Z. The amount of the time shift 
is determined by comparing the bolometer signal 
with the signal produced by a generator. This real 
generator signal takes the place of the hypothetical 
signal produced by an image at 1. The comparison 
is performed by an electronic phasemeter whose 
d-c output voltage is proportional to the image dis¬ 
placement A Z, and consequently to the guide angle 
ft. This voltage operates an amplidyne system which 
rotates the whole rangefinder about the point C in 
a direction such as to bring the optical axis M onto 
the target and reduce the guide angle to zero. 

Ranging 

When the automatic following device is operating 
correctly, the eye B is kept pointing directly at the 
target. The operator rotates the other eye A to the 





















THE INFRARED RANGEFINDER 


353 


proper parallax angle in order to make it also point 
directly at the target (see Figure 38). A second 
phasemeter compares the alternating signal from 
eye B with that from eye A. The d-c output voltage 
of this phasemeter is zero when both eyes are 
pointed correctly. The operator accomplishes the 
rotation of eye A by remote control and reads the 
amount of rotation from a dial on his instrument 
panel. In the present experimental model, the range 
is determined indirectly from the dial reading by 
means of a table. It would be possible to engrave 
ranges on the dial so that they could be determined 
directly, just as in the standard optical rangefinder. 

912 3 Description of the Component Parts 
The Optical System 

Figure 41 shows a photograph of the rangefinder, 
and Figure 42, a diagram. As shown in the diagram, 
the bolometers are actually not separated but are 
constructed coaxially on a single rotating shaft. 
This construction is advantageous because, if one 
bolometer were placed at each end of the range¬ 
finder, it would be necessary to provide for exact 
synchronization of their rotations. 

Radiation is focused on the bolometer by a fast 
parabolic mirror (12-inch diameter //1.0), repre¬ 
sented in the simplified diagram, Figure 40, lower, 
as a lens. The mirror marked A in Figure 42 points 
directly out to sea. A tangent screw at the end of 
the lever arm L is employed to change the angle of 
this mirror with respect to the rangefinder; i.e., to 


change the parallax angle. This tangent screw is 
remotely controlled with a selsyn type motor as 
explained earlier. The second mirror B points along 
the rangefinder to a double mirror (penta reflector) 
which redirects its view to sea. The penta reflector 
virtually separates the fixed eye B from the movable 
eye A by a 15-foot interocular distance. In the 



Figure 41. Photograph of rangefinder. 


photograph (Figure 41) the penta reflector is in the 
box at the left end and the unit at the right end 
contains the bolometers, parabolic mirrors, and 
associated gear. This unit at the right is referred 
to hereafter as the head. The photograph is taken 
looking at the front of the rangefinder. 

Description of the Bolometer 

The two squirrel-cage bolometers are mounted, 
one directly above the other, on the same rotating 
shaft. The various stages of bolometer construction 



* 

4- 

DIRECTION FROM WHICH PHOTOGRAPH (FIG 41) IS TAKEN 

1 

z - — - - - 




B 



,BOLOMETERS 










-J 

% 




ft 


■y! 

/ 

PAfU 

MIRF 

\| o a. 

T— 

Y 

* 

DOUBLE 

A MIRROR 

& 

t 

k 

v 

TANGENT r 
SCREW L 

~V—J\ 





Figure 42. Diagram of rangefinder. 



















































354 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


and assembly are shown beginning at the left in 
Figure 43, and a complete double bolometer appears 
at the extreme right. 

The strips are spaced at the rate of 24 for the 
full circumference of each bolometer; but of these, 
only 8 strips are actually present. The individual 
strip is soldered at each end to a flattened silver 
wire. The supporting silver wires are held between 
two concentric brass rings pressed onto the shaft. 
The wires are insulated from the rings by mica 
sheets. These rings and the shaft are shown at the 
left end of the ruler in Figure 43. 

At the next stage of construction (shown just 
below the 1-inch mark on the ruler), the outer brass 
ring has been cut out in the center of each bolometer 
to expose the silver wires beneath the outer* mica 
sheet. 

At the third stage (3% inches on the ruler) the 
outer brass ring has been turned to diameter, the 
silver wires have been cut through, and the inner 
brass ring has been relieved, by turning, to provide 
an open space for the strips. Each strip is suspended 


across this open space in such a way that it replaces 
that part of the silver wire which has been cut 
away. Eight of the 24 possible strip positions are 
utilized for active strips, and in the case of the 
upper bolometer, three of the possible strip positions 
are shorted to conduct the signal from the lower 
bolometer to the top of the shaft. The signal leads 
for both bolometers go out through six holes in the 
top end plate of the casing. 

In the center of Figure 43 at 5% inches on the 
rule are shown the various parts of the bolometer 
casing. The long cylindrical silver tube shown sec¬ 
ond from the bottom is slipped over both bolom¬ 
eters. The end plates shown above and below this 
tube are slipped over the ends of the shaft. 

In the next assembly (shown below the 9-inch 
mark on the ruler) the strips have been installed 
and covered with aluminum black, the casing is in 
place, apertures have been cut in the silver tube 
opposite each set of four strips, and a charcoal trap 
has been attached to the lower end of the shaft. 
The four apertures are covered with two AgCl win- 



Fioure 43. Figure showing various stages of bolometer construction. 












THE INFRARED RANGEFINDER 


355 


dows (protected from solarization by gilsonite). 
One unattached AgCl window is shown at the top 
center of Figure 43. The glass seal-off which does 
not show clearly in the photograph is located just 
above the charcoal trap. 

Amplifier System 

Bolometer Bridge. The plus and minus bolometer 
strips connected in series form two arms of an a-c 
Wheatstone bridge; the other two arms are pro¬ 
vided by the two halves of the primary winding of 
the input transformer. By connecting the two 
Wheatstone bridges in series a common battery can 
supply both bolometer circuits with current. This 
battery consists of two l^-volt dry-cell units in 
parallel. 

The strips which comprise the bridge arms are 
individually matched so that each has approxi¬ 
mately the same resistance (about % ohm). In the 
absence of radiation, the bridge is balanced. The 
opposite current flow in the two halves of the trans¬ 
former primary produce opposing fields in the core 
and prevent saturation. 

Strips of opposite polarity are placed adjacent to 
one another in the bolometer. Consequently, as the 
target image is scanned by the strips, an alternating 
unbalance of the bridge takes place. This alternat¬ 
ing voltage which appears across the primary wind¬ 
ing of the transformer induces an alternating cur¬ 
rent in the secondary winding which is carried by 
the line to the preamplifier in the instrument house. 
The turns ratio of the transformer is chosen to 
match the bolometer impedance (about 6 ohms) to 
the line impedance (500 ohms). 

Each bolometer consists of two groups of four 
active strips separated from each other by two 
groups of eight phantom strips. Consequently, when 
the bolometer scans past the target image the re¬ 
sulting transformer output signal consists of a 
coherent wave train of two full cycles followed by 
a four-cycle gap, and then another wave train of 
two more full cycles, etc. In other words, no signal 
is produced when the target image scans past the 
phantom strips. The amplifiers in the instrument 
house are sharply tuned and have a long time con¬ 
stant (about one second) so that the amplifier out¬ 
put is a continuous wave train. The bolometer rota¬ 
tion frequency is 200 rpm and the signal frequency 
is 40 c. 

Amplifier Circuit. The preamplifier is direct- 


coupled to the tuned amplifier so that it is con¬ 
venient to discuss both of these circuits together. 
The signal is brought to the preamplifier by the 
500-ohm line mentioned earlier. A transformer 
matches the impedance of this line to the grid of 
a pentode amplifier stage. The output of this stage 
is capacitance-coupled to the grid of a triode 
cathode-follower stage. A logarithmic attenuator is 
inserted between the pentode and the triode to serve 
as a gain control. 

The second stage of the preamplifier is directly 
coupled to the cathode of the input stage of the 
tuned amplifier. A common cathode impedance 
serves both stages. This input stage is another 
pentode amplifier the control grid of which is con¬ 
nected to the output of a twin-T feedback network. 
The pentode is direct-coupled to the grid of a triode 
cathode-follower stage. The output of this second 
tuned-amplifier stage is applied both to the input 
of the feedback network and directly to the cathode 
of the third stage. The third (pentode amplifier) 
and fourth (triode cathode-follower) stages are 
essentially a repetition of the first two stages with 
a separate twin-T feedback path to the grid of the 
third stage. The two twin-T units in series combine 
to produce a high-gain, sharply tuned, 40-cycle 
amplifier. The values of the circuit elements in the 
second unit (third and fourth stages) differ from 
the corresponding circuit elements in the first unit 
because the amplifier is direct-coupled throughout. 
The output is taken from a cathode-follower stage 
and has a correspondingly low output impedance. 
The twin-T networks are tuned with a gang of 
eight rheostats all mounted on a common shaft. 
The tuning range is from 38 to 42 c, and the band¬ 
width at 40 cycles is between one-half and two- 
thirds of a cycle. 

Phasemeter Circuits. Each tuned amplifier feeds 
directly into an amplifier-limiter, and each pair of 
amplifier-limiters feeds into a differentiator and 
trigger circuit. Such a combination of two amplifier- 
limiter channels followed by a differentiator and 
trigger circuit is referred to in this report as a 
phasemeter. A double-diode at the input of the 
amplifier-limiter circuit operates with fixed battery 
bias (±1.5 volts) to limit the signal. This limited 
signal is applied to the grid of a pentode amplifier 
which is operated at comparatively low gain. The 
output of this amplifier is limited again by a similar 
double-diode biased by the same source as the input 





356 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


stage. A triode follows the second limiter and is 
used as a phase-inverter. The output signal may be 
taken from either the plate or cathode in order to 
make available a 180-degree phase inversion, as 
needed. The signal proceeds through another low- 
gain pentode amplifier and double-diode limiter 
to the final output. The circuit is capacitance- 
coupled throughout. 

There are two inputs in the differentiator and 
trigger circuit, each of which is fed by a separate 
amplifier-limiter channel. The first stage in each 
case is a comparatively low-gain pentode amplifier. 
This is followed by a resistance-biased double-diode 
limiter stage. At this stage any usable signal has a 
good square waveform. The last limiter is followed 
by a differentiator network (RC = 25psec). The 
negative pulses from the two differentiator circuits 
operate the trigger circuit alternately. The trigger 
circuit, which comprises four pentodes, is an adapta¬ 
tion of the fundamental Eccles-Jordan circuit. A 
zero-center, 1-milliampere meter in the cathode 
circuit measures the average cathode-to-cathode 
potential difference. 

It is an important characteristic of the instrument 
that noise variations in the input signal average out 
to zero reading of the indicating meter if they are 
completely random. If both input signals consist 
solely of random noise, the meter will fluctuate 
about zero and will have a zero average value. If 
the noise is superimposed upon two signals having 
a non-zero phase difference, it will cause fluctuations 
about that value; but the average value of the read¬ 
ing will be a true measure of the phase difference 
between the two signals. However, if the noise has 
a phase coherence, that is, if the phase relation 
between the noise superimposed on one input signal 
and the noise superimposed on the other is not com¬ 
pletely random, then the phasemeter reading will be 
affected by the noise. It is important to eliminate 
all sources (microphonic, radiative, etc.) of non- 
random noise. 

Amplidyne Control Circuit. The output of the 
bearing phasemeter is taken to the amplidyne con¬ 
trol circuit. This is taken, as before, from the 
cathodes of the trigger tubes. 

The signal is fed to a push-pull power amplifier 
circuit. The amplidyne field winding is the plate¬ 
load impedance. The gain control is located in the 
cathode circuit. With zero input signal (both con¬ 
trol grids at the same potential) it is necessary to 


balance the plate currents so that the amplidyne 
meter will not run. This balance is accomplished by 
adjustment of the screen potentials with the poten¬ 
tiometer shown in the diagram. In order to ascer¬ 
tain (without turning on the amplidyne generator) 
whether the plate currents are properly balanced, 
a twin-indicator electron-ray tube (magic eye) is 
employed. This indicator is coupled to the ampli¬ 
dyne field winding by a double-triode cathode- 
follower stage which supplies the proper operating 
potentials to the indicator. It was mentioned earlier 
that the rangefinder lags a uniformly moving target. 
It is possible to compensate for such a lag by 
adjustment of the (screen) balancing potentiometer. 

The amplidyne control chassis also contains a 
selsyn-type receiver. It is geared to a dial graduated 
in degrees which indicates the true bearing of the 
rangefinder. A number of relays and selector 
switches permit the application of suitable signals 
to the push-pull grids for continuous scanning be- 
between selected bearing limits. Another circuit 
operates off the signal-level meter to allow auto¬ 
matic break-in. That is, a signal of a predetermined 
level causes the amplidyne circuit to cease scanning 
between limits and commence following automati¬ 
cally the target which produced the signal. 

The Representation Unit 

Leads brought out from the trigger tube cathodes 
in the range phasemeter are taken to the d-c ampli¬ 
fier. The d-c amplifier circuit has been adapted 
from a standard balanced-triode circuit by placing 
the load impedance in the cathode circuits. The 
Esterline-Angus 1-milliampere recorder operates 
with series resistance as a d-c voltmeter to measure 
the cathode-to-cathode potential. The recorder sen¬ 
sitivity can be adjusted by means of a rheostat, the 
sensitivity usually being adjusted so that one small 
division on the record chart corresponds to a motion 
of the tangent screw of 0.002 inch. The balancing 
potentiometer connected between the two cathode 
resistors can be adjusted to place the phase zero at 
any desired position on the record chart. 

Field Tests on the Infrared 
Rangefinder 

On October 3, 1944, an official demonstration of 
the rangefinder was held for NDRC and Service 
representatives. Both manual and automatic track- 



THE INFRARED RANGEFINDER 


357 


ing were employed and good results both in accu¬ 
racy of tracking and accuracy of range were ob¬ 
tained on the tug target. 

Tests on Controlled Target 

Operating Procedure. Figure 44 is a reproduction 
of typical records taken during a controlled target 
test on October 3, 1944. On the left is a record of 
the output of the range phasemeter, and on the 
right is a record of signal strength as received by 
the fixed eye B. 

The sensitivity of the phasemeter is adjustable. 
It has been adjusted so that one small lateral divi¬ 
sion on the range phase record is equivalent to a 
motion of the tangent screw of 0.002 inch. The 
phase zero, that is, the point on the scale that cor¬ 
responds to the case where both eyes are pointing 
directly at the target, is represented by the heavy 
line near the right border. The record begins at the 
bottom of the page, and each of the curved horizon¬ 
tal lines marks a 30-second time interval. 

The signal strength is shown on the right and is 
recorded logarithmically on a scale such that one 
small lateral division is equivalent to a change in 
signal level of 1 db. The average noise level is rep¬ 
resented by the line 5 db to the right of the left 
border. This record also begins at the bottom of the 
page and is synchronized in time with the phase¬ 
meter record. 

At the beginning of these records the rangefinder 
was fixed on the calibration source Dog, which 
consists of an electrical heater at the focus of a 
36-inch mirror located 5,000 yards from the range¬ 
finder at Nahant. From the record it is evident that 
the signal received from this source is 26 db above 
the average noise level. Since the range of this 
source is known to be 5,000 yards, the proper setting 
of the tangent screw can be calculated. When the 
tangent screw is adjusted to this position the range 
phasemeter record shows a constant deflection which 
is taken to be the phase zero, and the record has 
been marked accordingly. This deflection will result 
whenever both bolometers are receiving radiation 
from the target along their respective optical axes, 
i.e., whenever both eyes are looking directly at the 
target. The particular value of this zero deflection 
has no significance. It depends on several factors 
which will be discussed later in connection with 
bolometer construction. Means have been incorpo¬ 
rated within the phasemeter for arbitrarily chang¬ 


ing this zero to any desired point on the record; for 
example, the center. As mentioned earlier it is pos¬ 
sible to incorporate optical calibration means in the 
rangefinder. This would obviate the necessity of a 
distant calibration source, such as the Dog. 

After this calibration (which required 1 minute 
of time) the rangefinder was turned from the Dog 
to the controlled target. The target ship was the tug 
Francis C. Hersey of the Boston Tow Boat Com¬ 
pany. The target ship was to make a number of 
runs at approximately constant range. In each case 
the ship was to return over substantially the same 
course to its starting point before going to the next 
range. The records shown in Figure 44 correspond 
to one of these constant-range runs. Actually, the 
range varied from about 2,500 yards to about 3,000 
yards during this particular run. The rangefinder 
followed the ship automatically throughout the 
entire course. 

The signal-level record indicates that the signal 
was about 30 db above the noise level when the 
target was first picked up (time, 1% minutes). 
Thereafter, it increased slowly to a maximum of 
34 db (time, 3% to 4 minutes) and then decreased 
to about 27 db above the noise level (time, 7% 
minutes). During this time the ship had traveled 
about 1,500 yards and increased her range by about 
350 yards. Near the end of the records (time, 8 
minutes) the target ship turned around in order to 
reverse her course. This is indicated by the coding 
mark at the left edge of each record. When the ship 
turned, the change in aspect caused the signal level 
to fall to a point only 22 db above the average noise 
level. This influence of aspect on signal level is one 
explanation of the variable character of the signal- 
level record. However, the predominating cause is 
presumed to be the influence of the gustiness of the 
wind on stack temperatures. The steady character 
of the signal received from the Dog at the begin¬ 
ning of the record demonstrates that these varia¬ 
tions are not characteristic of the equipment. It 
should be noted that the range phase is independent 
of the signal level. This independence is particu¬ 
larly striking when the ship turned around (time, 
8 minutes) producing the strong dip shown by the 
signal-level record. 

When the rangefinder was moved from the Dog 
to the target ship (time, 1 minute) a large transient 
appeared on the range phase record. After this tran¬ 
sient died out the record stabilized at a point seven 








358 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 



TIME IN 
MINUTES 



Figure 44. Record made by rangefinder corresponding to a constant-range run. 
















































































































































































































































































































































































































THE INFRARED RANGEFINDER 


divisions .(14 thousandths) to the left of the pre¬ 
viously determined phase zero. At this time (time, 
1 % minutes) the tangent screw was still at its 
former value of 417 thousandths. The sign of this 
displacement of the range phase record from zero 
is an indication to the operator that the range of 
the ship is less than that of the Dog. Therefore, the 
parallax angle for the ship must be greater than 
that for the Dog, and the tangent screw displace¬ 
ment must be increased accordingly to return the 
phase record to zero. The notation M = 431 (time, 
2 minutes) indicates that the operator turned the 
eye until the tangent screw dial read 431 thou¬ 
sandths. It was permitted to remain at this value 
for the duration of the run. 

It will be observed that the fluctuations in the 
record with the present apparatus would make it 
difficult for the operator to maintain a constant 
phase zero reading by continual readjustment of 
the tangent screw. Consequently, it is customary 
for him to allow the tangent screw to remain fixed 
at some value which will keep the phasemeter rec¬ 
ord within a few thousandths of the phase zero. 
As the range of the target varies, the operator 
changes the position of the tangent screw from time 
to time in order to maintain this condition. The 
steady drift of this particular record from left to 
right is evidence that the range of the target is 
slowly increasing and that it will soon be necessary 
to readjust the tangent screw. 

At any moment when the record actually corre¬ 
sponds to the phase zero, the range may be calcu¬ 


359 


lated directly from the reading of the tangent screw 
dial. Whenever the record departs from zero, it is 
necessary to correct the reading of the tangent 
screw dial by the amount of this departure before 
calculating the range. An example of such a calcula¬ 
tion is shown on the record at a point 7 x / 2 minutes 
after the start of the run. 

At the left edge of the phase record there appears 
a column of visual ranges. These were obtained 
with a 1-meter Bausch and Lomb coincidence 
rangefinder (Mark 57). Interpolation of these visual 
ranges for the points 5 and 9 minutes after the start 
of the run yields a value of 2,875 yards for the 
point 7i/ 2 minutes after the start. At this point the 
FIR rangefinder gave a value of 2,880 yards. 

A set of visual and infrared range findings were 
recorded simultaneously during the tests on this 
night, and these are set down in Table 10. The 
result may briefly be summarized by stating that 
the infrared ranges varied within about 10 per cent 
of the ranges determined visually. 

Performance of Automatic Following 

The automatic following equipment operates as 
a continuously variable closed system. The discus¬ 
sion of this subject under “Automatic Following” 
and “Ranging” in Section 9.12.2 explains how the 
fixed eye B is used to measure the guide angle 5. 
As a result of this measurement the bearing phase¬ 
meter produces a d-c voltage which is directly pro¬ 
portional to 5. This d-c voltage is fed into an ampli- 
dyne system which in turn rotates the rangefinder 


Table 10. 


Time 

(minutes) 

M 

(thousandths) 

M corrected 
(thousandths) 

R IR 

(yards) 

VIS 

(yards) 

Error 

(yards) 

Error 
(per cent) 

V/2 

14 

431 

2700 




2 

2 

433 

2520 




2V 2 

4 

435 

2390 




3 

4 

435 

2390 

2650 

-260 

9.8 

3% 

3 

434 

2460 

2680 

-220 

8.2 

4 

4 

435 

2390 

2710 

-320 

11.8 

4% 

3 

434 

2460 

2730 

-270 

9.9 

5 

2 

433 

2520 

2750 

-230 

8.4 

5% 

-1 

430 

2790 

2775 

15 

0.5 

6 

-3 

428 

2990 

2880 

190 

6.8 

6V 2 

-3 

428 

2990 

2825 

165 

5.8 

7 

-3 

428 

2990 

2850 

140 

4.9 

7 y 2 

-2 

429 

2880 

2875 

5 

0.2 

8 

-3 

428 

2990 

2900 

90 

3.1 

8V 2 

-4 

427 

3100 

2925 

175 

9.1 

9 

-5 

426 

3220 

2950 

270 

9.2 












360 


FAR INFRARED RECEIVING SYSTEMS FOR MILITARY APPLICATIONS 


with a velocity which is directly proportional to the 
d-c voltage. The rotation of the rangefinder changes 
the guide angle 5. The significance of these rela¬ 
tions is that the velocity of rotation of the range¬ 
finder is proportional to the guide angle 5. 

The relations discussed in the preceding para¬ 
graph have been oversimplified by neglecting the 
time lags. The electronic portion of the system is 
characterized by a relaxation time of the order of 
1 second due principally to the sharply tuned 
amplifiers. This means that when the guide angle 5 
changes, the d-c voltage lags this change by an 
amount approximately equal to the relaxation time. 
The amplidyne and mechanical portions of the sys¬ 
tem are also characterized by a finite relaxation 
time. This means that when the d-c voltage changes, 
the rangefinder rotation velocity lags the change in 
the d-c voltage. The effect of these time lags is 
cumulative. The d-c voltage corresponds not to the 
actual 5 but to the guide angle of an earlier time. 
The rangefinder velocity corresponds not to the 
actual d-c voltage, but to the voltage that was 
present at an earlier time. Therefore, the range¬ 
finder velocity is not directly proportional to the 
guide angle, but corresponds to the guide angle of 
a cumulatively earlier time. As a result, the auto¬ 
matic following system may exhibit a transient 
hunting period characterized by an exponentially 
damped oscillatory wave. 

The rangefinder, when it is following a target 
which moves uniformly in bearing angle, lags it by 
a constant angle which is proportional to the rate 
of change in bearing of the target. This lag is evi¬ 
dent to the operator on the bearing phasemeter. 
Electric compensation can be introduced by the 
operator to make the rangefinder follow the target 
exactly. The amount of this lag on the night of 
October 3, when the tug was moving laterally with 
a speed of 10 knots at a range of 2,000 yards, was 
50 degrees in phase, which amounts to a guide angle 
of about ten minutes of arc. 

An inherent defect of the 24-strip, squirrel-cage 
bolometer is the presence of multiple horizontal 
fields; that is, bearing phasemeter readings are the 
same for several angular positions of the target. 
These multiple fields are a result of the fact that 
several image positions on the bolometer give a 
signal with the same phase relation to the generator 
signal and the amplidyne system will follow on any 
one of the zero-phase points. It may also be made 


to follow at any intermediate point by the intro¬ 
duction of suitable bias and phase shifts. In order 
to locate an invisible target in the central field, it is 
necessary for the operator to scan deliberately across 
the outer fields before changing the servo control 
from scan to follow. Thereafter, the amplidyne sys¬ 
tem will follow at the point O. 

The number of strips exposed to the target field 
is proportional to the total number of strips. If the 
number of strips were reduced from 24 to 6, the 
number of horizontal fields would be reduced from 
5 to 1%. 

The amplidyne following mechanism operates on 
receding targets until the signal is almost obscured 
by noise. On the night of October 3, the tug 
Francis C. Hersey was automatically followed to 
a range of about 7,500 yards before it was lost. On 
the night of October 19, targets were followed suc¬ 
cessfully for several minutes when the signal was 
apparently equal to the noise. Under these condi¬ 
tions the rangefinder fluctuated about the target 
position with an amplitude of about 10 minutes 
of arc. 

Similar tests were made on the nights of Septem¬ 
ber 8 and 11 and October 2 and 19. These addi¬ 
tional tests gave range accuracies and automatic¬ 
tracking performance equivalent to those obtained 
on the night of October 3 at the official demon¬ 
stration. 

912,5 Present Status 

The successful tests carried out on the infrared 
rangefinder have led to a further development of 
the device. The New York Navy Yard has under¬ 
taken the engineering development required to adopt 
the present model of the rangefinder for shipborne 
use and for the construction of a pilot model. Con¬ 
sultation for this work was provided by the Harvard 
contract. 

913 SPECTROPHOTOMETRIC ELEMENT, 
TYPE T [SETT] 

Introduction 

The development of a detector, for use in the 
infrared spectrometer for the recording of infrared 
spectra, which would be free from thermal drift and 
uninfluenced by vibrations was an item of extreme 
importance to industrial concerns involved in the 


~ TtE8TRXCTt:H 




SPECTROPHOTOMETRIC ELEMENT, TYPE T [SETT] 


361 


production of aviation gasolines, synthetic rubbers, 
and other chemicals used in the successful prosecu¬ 
tion of the war. Previous methods employed sensi¬ 
tive thermocouples in conjunction with a high- 
sensitivity galvanometer relay. This method, 
although highly successful in the past in university 
laboratories, has the objection that the recorder 
may drift and that the galvanometer relay reacts to 
shock and building vibration. 

The above problem has been satisfactorily dealt 
with by BTL workers under Contract OEMsr-1098, 
who have developed, for spectrographic use, a ther¬ 
mistor bolometer and an electronic amplifier for 
spectroscopic recording purposes. This work is de¬ 
scribed in their final report. 25 

913,2 Description of Component Parts 

Bolometer 

The bolometers developed were made of ther¬ 
mistor material which is a semiconducting substance 
with a negative temperature coefficient of resistance. 
Essentially, two types of bolometers were devel¬ 
oped, namely, the unbacked (also known as air- 
backed) and the backed units. Description and 
performance characteristics of these are given in 
considerable detail in Section 8.7. The backed units 
were mounted on various materials such as rock 
salt, quartz, and glass and had the advantage that 
they were more rapidly responding than the un¬ 
backed units. At higher chopping frequencies these 
were generally more sensitive than the unbacked 
units. 

The dimensions of the bolometer area are, in 
general, about 0.2x3.0 millimeter. They are 
mounted in a thin cylindrical case behind a protect¬ 
ing window of rock salt or, in some cases, silver 
chloride. The resistances of these bolometers is of 
the order of 2.7 megohms, and the unit must there¬ 
fore be regarded as a high-impedance bolometer. 
The unbacked units exhibit a frequency response 
which can be described by a time constant. These 
time constants are of the order of 135 milliseconds. 
The frequency response of the backed units is not, 
strictly speaking, one which can be characterized 


by a time constant. For the frequency interval 
within which they are designed to operate the re¬ 
sponse behaves as though they had a time constant 
of from about 3 to 6 milliseconds. 

Amplifier 

The bolometer and a compensating strip, which 
may be either another thermistor strip or a metallic 
resistor, are made the two arms of a Wheatstone 
bridge. The bolometer bridge feeds into a preampli¬ 
fier employing three vacuum tubes. The preampli¬ 
fier, in turn, feeds into the main amplifier which 
contains a twin-T network tuned to 15 c. This part 
of the amplifier contains three stages of amplifica¬ 
tion. Following the amplifier is a rectifying system 
which furnishes a d-c current which may be used 
to operate a recording milliammeter. When radiation 
falls on the bolometer its resistance changes and 
the balance of the bridge is disturbed. It is the 
unbalance of the bolometer bridge which is ampli¬ 
fied, rectified, and ultimately recorded. 

9133 Performance 

The BTL detector developed for spectroscopic 
recording has been tested at the University of 
Michigan and its performance is reported in two 
NDRC reports. 26 ’ 27 It may be stated in a general 
way that for industrial infrared spectrophotometry 
the SETT detector is satisfactory. Its performance, 
while not entirely as sensitive, is comparable to 
that of a thermocouple and galvanometer relay. It 
is found that it will detect a signal of about 0.004 
pw above the noise of the detecting system. The 
SETT device has the definite advantage over a 
thermocouple and galvanometer relay system that 
it is virtually drift-free and that its time of response 
is much shorter. 

In certain regions of the spectrum the thermistor 
material transmits rather than absorbs the radiant 
energy falling upon it. In such regions the device 
is not so sensitive as would be desirable. This de¬ 
fect might, of course, be remedied if a blacking 
material could be applied which would not essen¬ 
tially alter the resistance of the BTL bolometer. 










APPENDIX 


XOTES ON GENERALIZED PHOTOMETRY, 

WITH PARTICULAR APPLICATION TO THE NEAR INFRARED * 

Revised Schematic Version 

By G. A. Van Lear. Jr. 

Foreword 


The following sections present the near-infrared 
nomenclature and units approved and recommended 
bv the Combined NAN Committee of the C.C.B. on 

JoD€ 10. 1^43. 

Tfce formulation so approved culminates devel¬ 
opments represented while in progress by NDRC 
Reports 16.-I-5 *0SRD No. 13S4 and contributed 


to by numerous British and American scientists 
working in the field. While Report 16.4-9 is hereby 
superseded. Chapters I and II of Report 16.4-5 may 
be found useful in elaboration. Chapters III and IV 
of that report should be disregarded, particularly be¬ 
cause of an essential difference in the infralumen as 
defined there and here. 


Symbol* Not Defined in Table* I, II 
♦ Generally following Illuminating Engineering 
Nomenclature and Photometric Standard*. Illumi¬ 
nating Engineering Society. 51 Madison Ave.. New 
York 1942. > 

E Illumination 
F Luminous flux i lumens) 

H Irradiancy. i.e.. incident radiant flux per unit 
area * watt cm 2 ) 

Hi Spectral irradiancy. i.e.. incident spectral radi¬ 
ant fiux per unit area (watt cm 2 per micron ) 
k\ Relative luminosity factor 
4 Wavelength • microns! 

Radiant flux. i.e.. radiant power i watts) 

^ Spectral radiant flux »watts per micron» 

*XDHC Report 16.4-10 tOSRD No. 15S5> under NDRC Contract XDCre-lS5 with Cniveratr of Michigan. Ann 
Arbor. Miebiaan 


363 


Table I. Detectors, receivers, and sources. 


364 


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367 


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368 


APPENDIX 


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which will usually be nearly unity, and 

where f is the effective holo-infra re¬ 

sponse factor of Table 1. 



























GLOSSARY 


a. Absorptivity, the ratio of resistance change per watt of 
heat power input to resistance change per watt of elec¬ 
trical power input. 

ACW. Average clear weather; transmission 0.6 per sea 
mile in NIR; 1 cm ppt H,>0 per 1.000 yd in IIR. 

ACW Range. Operational range in ACW; voice, at night, 
unless otherwise indicated. 

All-round System. System with hpi and hpr widths over 
about 100 degrees. 

Background Noise. Spurious erratic response produced in 
receiving instruments due to variation in background. 

Background Signal. Spurious steady response produced 
in receiving instruments due to variation of background. 

Bare Cell. Detector cell without optical system. 

Bare Source. Source alone without optical system. 

Beam Candlepower. The apparent intensity of a projector 
when viewed from a point such that the entire optical 
aperture appears bright and the illumination varies in¬ 
versely as the square of the distance from the projector. 

Beam Spreader. An optical element for imparting an 
angular divergence of a few degrees to a collimated inci¬ 
dent beam. 

Black Body. A body which absorbs and radiates all wave¬ 
lengths according to Planck’s law. 

Bolometer. Heat sensitive device depending for its sensi¬ 
tivity upon the change of resistance with temperature. 

Breakdown Potential. The steady d-c voltage required 
to initiate an electric discharge through a flash lamp. 

Brightness. The intensity of an element of surface 
divided by the area of the element projected on a plane 
perpendicular to the direction of the measurement. 

Brilliance. The concept of brightness extended to include 
radiation at wavelengths beyond the visible region. 

BTL. Bell Telephone Laboratories. 

BuAer. Bureau of Aeronautics, Navy Department. 

BuShips. Bureau of Ships, Navy Department. 

CAM. Cloud attenuation meter. 

C h . Heat dissipation constant. 

Cesium Vapor Lamp. A low-voltage arc lamp for the infra¬ 
red similar to the sodium vapor lamp for the visible 
region. 

Cm Ppt H 2 0. The length of the cylinder produced if all 
the water vapor in a column along the absorbing path 
were condensed into water. 

CMU. Control meter unit. 

Concentrated-Arc Lamp. A low-voltage arc lamp having 
nonvaporizing electrodes sealed in an atmosphere of 
inert gas, with a small, brilliantly incandescent cathode 
spot. 

Continuum. A spectrum having no discontinuities in the 
wavelengths at which radiation is emitted. 

Cosine Law. The law in accordance with which the 
brightness of a perfectly diffusing surface varies in any 
direction as the cosine of the angle between that direc¬ 
tion and the normal to the surface. 

CRO. Cathode-ray oscilloscope. 

c-w. Carrier wave. 

Daylight Visible Range. Limit range at which a large 


black object can be seen against white sky under given 
weather conditions. 

A/. Increment of frequency. 

Duplex (operation). Transmitter and receiver simul¬ 
taneously operable. 

ehT. Effective holotransmission (see the Appendix). 
ehT°. Standard effective holotransmission with reference 
to radiation from a tungsten source at 2848 K color 
temperature, and to any specific radiation detector (see 
the Appendix). 

ENI. Equivalent noise input. 

Entrance Angle. Solid angle within which a beam must 
enter the base in order for a triple mirror to have the 
retrodirective property. 

Entrance Pupil. Effective gathering area of receiver opti¬ 
cal system. 

Equivalent Holocandle. Unit of intensity in the holo- 
system (see the Appendix). 

bvT. Effective visual fractional transmission (of a filter). 
Exit Pupil. Effective area from which transmitter beam 
emerges. 

F. Bridge factor. 

Far Infrared. Radiation of wavelengths greater than 1.5 
microns. 

Field of View. Solid angle “seen” by the detector. 

FIR. Far infrared. 

FIRBARR. Far infrared bombsight with angular rate 
release. 

FIRR. Far infrared rangefinder. 

Flash Lamp. A gaseous discharge lamp for producing 
radiation pulses rather than continuous emission. 
//number. 1 /relative aperture where relative aperture is 
the ratio of the focal length to the diameter of the lens 
or mirror. 

G. Heat power absorbed by heat sensitive element in 
. watts per square centimeter. 

G f . Amplifier gain. 

G m . Maximum amplifier gain within band pass of ampli¬ 
fier. 

GPI. Glider-position indicator. 

H. Effective heat capacity of bolometer. 
hcp. Holocandlepower. (See the Appendix.) 
hlm. Hololumen. (See the Appendix.) 

Holo. The prefix used in the holo-system (defined in the 
Appendix) to denote its relation to the complete spec¬ 
tral emission from a tungsten lamp at color tempera¬ 
ture 2848 K as the primary holophotometric standard. 
Holocandlepower. (See the Appendix.) 

Hololumen. (See the Appendix.) 

hpi. Half-peak intensity (width of transmitter beam). 
hpr. Half-peak response (width of receiver directivity 
pattern). 

I. Current. 

I h . Bolometer current. 

IIR. Intermediate infrared. 

IIRR. Intermediate infrared receiver. 

Image Tube. Device converting NIR image to visible 
image. 


369 


370 


GLOSSARY 


Intermediate Infrared. Spectral region 1.4 to 6 microns. 

IRRAD. Infrared range and direction detecting equip¬ 
ment. 

JAPIR. Japanese infrared detecting system (see Section 
5.5). 

“Lock-in” System. Tracking system which operates after 
it is once pointed toward the distant source. 

MDS. Minimum detectable signal. 

MENI. Minimum equivalent noise input; heat signal 
necessary to produce rms voltage equal to rms Johnson 
noise of detector alone. 

Message Security. Freedom from interception of intel¬ 
ligible signals by the enemy. 

Microbeacon. Portable constant source emitting faint 
coded light for testing receivers. 

Microflash Lamp. A flash lamp which emits radiation 
pulses having a duration of the order of 1 microsec¬ 
ond. 

Microflux Source. A calibrated source for variable ra¬ 
diation signals of the order of 1 microhololumen. 

Minimum Detectable Radiant Power. Smallest radiant 
power detectable above the noise. 

Minimum Detectable Signal. Smallest signal detectable 
above the noise. 

MIT. Massachusetts Institute of Technology. 

Modulation Efficiency. Ratio of maximum useful crest- 
to-trough variation of modulated flux to average d-c 
flux with modulating device removed. 

Modulation Ratio. The quotient of per cent radiation 
modulation divided by per cent current modulation for 
an electrically modulated source. 

NAMU. Naval Air Modifications Unit. 

Narrow-Angle System. System with hpi and hpr widths 
less than 5 degrees. 

Near Infrared. Primarily the spectral region comprising 
wavelengths between 0.8 and 1.5 microns. 

Nernst Glower. Semiconducting incandescent electric 
heat source. 

NIR. Near infrared. 

NRL. Naval Research Laboratory. 

NVR. Visual range limit of a transmitter to the dark- 
adapted “standard” eye (see Chapter 2) in total dark¬ 
ness. 

a). 2 jt X frequency. 

ONRD. Office of Naval Research and Development. 

Operational Range. Observed limit range of a communi¬ 
cation system in field test. 

Optics Factor. Ratio of maximum transmitter intensity 
to maximum bare source intensity; or of maximum re¬ 
ceiver response to maximum bare cell response in field 
of uniform illumination. 

OSU. Ohio State University (Contract OEMsr-987). 

P. Power in watts. 

PbS Cell. Lead sulfide photoconductive cell. 

P-G System. Plane unit, electrically modulated, of plane- 
to-ground communication system. 

Photomultiplier Tube. A combined phototube and sec¬ 
ondary emission amplifier contained in a single evacu¬ 
ated glass envelope. 

Pip. Signal on cathode-ray oscilloscope screen seen as a 
very sharp deviation from the normal signal. 


Plan-Position Indicator. An all-round signal presenta¬ 
tion system in which target range and azimuth appear 
as radius and angle coordinates in a polar coordinate 
system centered on the screen of the cathode-ray tube. 

PND. Portable infrared detector. 

P-P. Plane-to-plane communication system. 

PR. Plane-to-plane recognition system. 

Press-to-Talk Button. A send-receive switch which is 
normally in the “receive” position. 

PSD. Portable ship detector. 

R. Resistance. 

Radiation Detector. The element or device, e.g., photo¬ 
tube, bolometer, in which radiant energy is converted 
into electric energy. 

R h . Bolometer resistance. 

RCA. RCA Victor Division of Radio Corporation of 
America. 

Receiver Efficiency. Ratio of flux received on detector 
cell to flux incident on receiver entrance pupil. 

Responsivity. Response in absolute units to incident 
radiation in watts per square centimeter. 

Retrodirective Mirror. See triple mirror. 

Retrodirective Reflector. A reflector which redirects in¬ 
cident flux more or less accurately back toward its point 
of origin. 

RMA. Radio Manufacturers’ Association. 

RMU. Reflector-modulator unit. 

R g . Balancing resistance. 

RTL. Retrodirective target locator. 

Scanning Head. A unit containing the optical system of 
both a transmitter and a receiver, aligned with coexten¬ 
sive or overlapping fields of view. 

SDU. Source-detector unit. • 

Send-Receive Operation. Transmitter and receiver oper¬ 
able only alternately. 

Sensitivity. See responsivity. 

SETT. Spectrophotometric element, type T. 

Signal Equivalent of Noise. A measure of the minimum 
signal that can be detected by an equipment due to in¬ 
herent noise limitations, more rigorously defined in the 
Appendix. 

Sine Law. The law in accordance with which the inten¬ 
sity of a linear source in any direction varies as the sine 
of the angle between that direction and the axis of the 
source. 

Smoothing Time. Period elapsing while filter removes 
fluctuations in output current of a vacuum-tube rectifier 
or direct-current generator. 

S/N Ratio. Signal-to-noise ratio measured in decibels. 

SND. Scanning infrared detector. 

SONRD. Secretary, Office of Naval Research and Devel¬ 
opment. 

SSD. Stabilized ship detector. 

SSL. Submarine Sound Laboratory. 

Stable Table. Gyroscopically stabilized horizontal plat¬ 
form. 

Standard Eye. Observer having red and infrared luminos¬ 
ity functions. (See Chapter 2.) 

System Security. Freedom from interception of any sig¬ 
nals by the enemy (as distinguished from message se¬ 
curity). 





GLOSSARY 


371 


T. Temperature. 

t. Time constant. 

TF Cell. Thallous sulfide photoconductive (Cashman) 
cell. 

Thermistor. Semiconducting material with large nega¬ 
tive temperature coefficient. 

Threshold Flux. Minimum rms variation of flux re¬ 
quired to give 100 per cent sentence intelligibility. 

TMR. Thermal map recorder for ground survey. 

Tracker System. System which automatically follows a 
distant radiation source. 

Transceiver Head. Head containing both transmitter and 
receiver optical systems and associated parts. 

Transmission Window. Spectral region where infrared 
radiation is transmitted. 

Transmitter Efficiency. Ratio of flux emerging in trans¬ 
mitter beam to total flux emitted by source. 

Triple Mirror. Triangular glass pyramid with 54-54-90° 
triangular faces and 60-00-60° base; a light beam enter¬ 
ing the base from any direction within a certain entrance 
angle is totally reflected three times to return accu¬ 
rately back on its original path. 


Type L. Thermal receiver with remote indicator. 

Type 10. A triggered flash lamp characterized by a high 
peak intensity and relatively long duration of its flash. 

Type 200 . A triggered microflash lamp of high-peak inten¬ 
sity for use in ballistic photography. 

Type 300. A self-breakdown microflash lamp having ex¬ 
tremely long life at flashing rates up to 120 per second. 

Vacuum Range. Maximum range of a communication 
system computed for conditions of no atmospheric 
attenuation. 

V h . Bolometer voltage. 

Very Narrow Angle System. System with hpi and hpr 
widths less than 1 degree. 

V p . Peak value of V b . 

W. Wave form factor. 

Wide-Angle System. System with hpi and hpr widths 
between 5 and 40 degrees. 

Window. Spectral region where infrared radiation is 
transmitted. 

Work Function. The amount of energy required to 
extract an electron from a given material, commonly 
stated in electron volts. 









BIBLIOGRAPHY 


Numbers such as Div. 16-302.2-M3 indicate that the document listed has been microfilmed and that its title appears in 
the microfilm index printed in a separate volume. For access to the index volume and to the microfilm, consult the Army 
or Navy agency listed on the reverse of the half-title page. 

Chapter 1 


1. Notes on Generalized, Photometry with Particular Appli¬ 
cation to the Near Infrared (Revised Schematic Ver¬ 
sion), G. A. Van Lear, Jr., OSRD 1585, NDCrc-185, 
Report 16.4-10, University of Michigan, June 16, 1943. 
(Reprinted as Appendix to this volume.) 

Div. 16-302.2-M3 

2 . Development of a Photoelastic Shutter for Modulating 
Infrared Light at Audiofrequencies, Hans Mueller, 
OSRD 1296, OEMsr-576, MIT, Jan. 1. 1943. 

Div. 16-301,2-Ml 

3. Development of a Photoelastic Shutter for Voice Com¬ 
munication over a Beam of Infrared Light, Hans Muel¬ 
ler, OEMsr-576, Special Report 16.4-47 MIT, Jan. 6, 
1946. 

4. Development of an Infrared Glider Position Indicator, 

G. A. Van Lear, Jr., and L. N. Holland, OSRD 4752, 
NDCrc-185, Service Project AC-56, University of 
Michigan, Jan. 11, 1945. Div. 16-320.1-M2 

5. Preliminary Development and Tests of a Retrodirective 
Locator for the Visible and Near Infrared, W. L. Hole, 
OSRD 3345, NDCrc-185, Report 16.4-22 (Report 16.4-21 
included as appendix), University of Michigan, Feb. 19. 

1944. Div. 16-320.2-MI 

6 . A Portable Hand-Held Infrared Optical Telephone, 

Harvey E. White, Alfred Einarsson, and Charles G. 

Miller, OSRD 6002, OEMsr-1073, Service Projects AC- 
226.03 and NS-371, University of California, Nov. 30. 

1945. Div. 16-303.1-M2 

7. German I-R Speech Communication Apparatus Li-80, 
C. V. Kent and W. L. Hole, NDCrc-185, Special Report 
16.4-1, University of Michigan, Sept. 11, 1943. 

Div. 16-303.3-MI 

8 . German Lichtsprecher 250/130, A. H. Nethercot, Jr. and 
Walter S. Huxford, OSRD 6200. OEMsr-990, Service 
Projects SC-126, SC-128, and SC-129, Report 16.4-70, 
Northwestern University, Oct. 20, 1945. 

Div. 16-303.3-M4 

9. Japanese Light Beam Telephone, Nikko 130 and 186, 

A. H. Nethercot, Jr. and Walter S. Huxford, OSRD 
5763, OEMsr-990, Service Projects SC-126, SC-128, and 
SC-129, Report 16.4-46, Northwestern University, Aug. 

I , 1945. Div. 16-303.3-M2 

10. A Photocell Test Set, W. L. Hole and L. N. Holland, 

OSRD 5298, NDCrc-185, Service Projects SC-5, NS- 
225, and NS-151, Final Report 16.4-35 covering the 
period from November 1, 1943 to July 19, 1944. Uni¬ 
versity of Michigan, Apr. 4, 1945. Div. 16-302.1-MI 

11. Report on an Infrared Recognition Device Developed 
at the University of Michigan and Demonstrated at 
Norfolk, Virginia, on February 12, 1943, P. H. Geiger. 

J. G. Black, and E. F. Barker, OSRD 1654, NDCrc-185, 


Report 16.4-12, University of Michigan, Apr. 1, 1943. 

Div. 16-304.2-M2 

12 . An Infrared Radiation System for Recognition and 
Ship-to-Ship Communication, J. G. Black and P. H. 
Geiger, OSRD 3546, NDCrc-185, Report 16.4-23, Uni¬ 
versity of Michigan, Mar. 1, 1944. Div. 16-304.1-MI 

13. A Near Infrared System for Recognition and Ship-to- 
Ship Communication, Type D-2, J. G. Black, P. H. 
Geiger, and A. F. Fairbanks, OSRD 5993, NDCrc-185, 
Service Project NS-151. Final Report 16.4-59, covering 
period from March 1, 1944 to October 31, 1945, Uni¬ 
versity of Michigan, Oct. 31, 1945. Div. 16-304.1-M2 

14. Plane-to-Plane Recognition, P. H. Geiger and J. G. 

Black, OSRD 5994, NDCrc-185, Service Projects NA- 
194 and AC-101, Final Report 16.4-60, covering period 
from July 1, 1944 to October 31, 1945, University of 
Michigan, Nov. 1, 1945 Div. 16-304.2-M3 

15. The Development of Apparatus for the Detection of 

Night Bombing Planes by Near Infra-Red Radiation, 
O. S. Duffendack, NDCrc-185, Engineering Research 
Project M-341, D-3 Progress Report 244, University of 
Michigan, June 1, 1942. Div. 16-304.2-MI 

16. The High Intensity Flash Lamp, Type 10, Development 
and Characteristics, W. W. McCormick and L. Madan- 
sky, OSRD 1934, NDCrc-185, Report 16.4-14, Univer¬ 
sity of Michigan, Aug. 24, 1943. Div. 16-301.11-M2 

17. The High Intensity Flash Lamp, Type 10, W. L. Hole 

and W. W. McCormick, OSRD 5297, NDCrc-185, Final 
Supplementary Report 16.4-34, covering period from 
August 25, 1943 to March 1, 1945, University of Michi¬ 
gan, Oct. 1, 1945. Div. 16-301.11-M5 

18. The Microflash Lamp, Type 200, Development and 

Characteristics, W. W. McCormick and L. Madansky, 
OSRD 1939, NDCrc-185, Report 16.4-13, University of 
Michigan, Aug. 21, 1943. Div. 16-301.11-MI 

19. The Type 200 Microflash Lamp and a Microflash Unit 

for Ballistic Photography, W. W. McCormick and A. F. 
Fairbanks, OSRD 5295, NDCrc-185, Service Project 
OD-147, Final Report 16.4-32, covering the period from 
January 1, 1943 to February 28, 1945, University of 
Michigan, Apr. 19, 1945. Div. 16-301.11-M3 

20. A Device for Tripping the Camera Shutter in High 

Speed Photography, A. F. Fairbanks, NDCrc-185, Spe¬ 
cial Report 16.4-44, University of Michigan, Nov. 1, 
1945. Div. 16-301.2-M3 

21. Adaptation of the Type 10 Flash Lamp to DeBrie 

Camera, A. F. Fairbanks, OSRD 5996, NDCrc-185, 
Service Project OD-173, Report 16.4-62, covering the 
period from February to September 1944, University of 
Michigan, Nov. 1, 1945. Div. 16-301.11-M6 

22. The Development of a Flashlamp Source for an Infra¬ 
red Range and Direction Apparatus, W. W. McCormick 




373 




374 


BIBLIOGRAPHY 


and W. L. Hole, OSRD 5296, NDCrc-185, Service 
Projects CE-22 and NR-103, Final Report 16.4-33, 
covering period from October 1, 1943 to June 15, 1945, 
University of Michigan, Aug. 1, 1945. 

Div. 16-301.11-M4 

22a. Ibid., pp. 5-7. 

23. Concentrated-Arc Lamps, E. C. Homer, OSRD 5300, 

OEMsr-984, Service Project NS-159, Final Report 16.4- 
43, covering period from May 12, 1943 to September 1, 
1945, Western Union Telegraph Company, Aug. 31, 
1945. Div. 16-301.12-M2 

23a. Ibid., p. 31. 

24. Infrared Voice and Code Communication Systems, 
Navy Type E, Walter S. Huxford and B. J. Spence, 
OSRD 5999, OEMsr-990, Service Project NS-159, Final 
Report 16.4-65, covering the period from May 1943 to 
October 1945, Northwestern University, Oct. 31, 1945. 

Div. 16-303.2-M5 

24a. Ibid., pp. 15-24. 

24b. Ibid., pp. 93-99,106. 

24c. Ibid., pp. 99^100. 

24d. Ibid., Appendix D., pp. 161-171. 

25. The Concentrated Arc-Lamp, a technical bulletin issued 
by the Western Union Telegraph Company, Water 
Mill Laboratory, Water Mill, L. I., N. Y. 

Div. 16-301.12-M3 


26. Transmitter and Receiver Optics, Point Source Voice 
System, John R. Platt, OEMsr-990, Special Report 

16.4- 14, Northwestern University, June 22, 1944. 

27. Theoretical Aspects of Optical Communication Sys¬ 
tems, Hartland S. Snyder and John R. Platt, OSRD 
6289, OEMsr-990, Service Project NS-159, Report 16.4- 
72, Northwestern University, Oct. 31, 1945. 

Div. 16-303.1-MI 

28. Infrared Voice Communication Systems for Aircraft, 

Everett W. Lothrop, Jr., John R. Platt, and Wallace R. 
Wilson, OSRD 6000, OEMsr-990, Service Projects AC- 
226.03 and AC-226.04, Final Report, covering the period 
from March 1945 to October 1945, Northwestern Uni¬ 
versity, Oct. 31, 1945. Div. 16-303.2-M6 

28a. Ibid., pp. 17-21. 

29. Near Infrared Transmitting Filters, R. C. Lord, OSRD 
3771, NDRC Division 16, June 1, 1944. 

Div. 16-301.3-M2 

30. Infrared Communication System, B. J. Spence, OSRD 
6001, OEMsr-1391, Service Project NS-243, Report 

16.4- 67, Northwestern University, Oct. 10, 1945. 

Div. 16-303.3-M3 

31. Report of Captain Guy Touvet to Bureau of Ships, 
Sections A to 0 inclusive, Naval Intelligence Transla¬ 
tions, April 15, 1940 to October 17, 1945. 


Chapter 2 


1. Near Infrared Transmitting Filters, Richard C. Lord, 
OSRD 3771, NDRC Division 16, June 1, 1944. 

Div. 16-301.3-M2 

la. Ibid., pp. 38-40. 

2. A Photocell Test Set, OSRD 5298, NDCrc-185, Service 
Projects SC-5, NS-225, and NS-151, Final Report 16.4- 
35, covering period from November 1, 1943 to July 19, 
1944. University of Michigan, Apr. 4, 1945. 

Div. 16-302.1-MI 

3. Near Infrared Filter Investigation, Report H-2199, Na¬ 
val Research Laboratory, Nov. 26, 1943. 

3a. Ibid., p. 18 ff. 

4. Near Infrared Filters, Engineer Board, Corps of Engi¬ 
neers Report 968, Feb. 1, 1946. 

4a. Ibid., p. 20 ff. 

5. Filter Comparator, Visual, Report ARL-N3/E.130, Ad¬ 
miralty Research Laboratory, Great Britain, Sept. 28, 
1943. 

6 . Range of Visibility of Infrared Sources with the Naked 

Eye and with Binoculars, Carl W. Miller and Lloyd N. 
Beck, OSRD 5739, OEMsr-1229, Report 16.1-106, Brown 
University, Oct. 25, 1945. Div. 16-121.2-MI 


7. The Transmission of Radiation, OEMsr-1085, OSRD 

Special Report 16.4-3, Polaroid Corporation, November 
1943. Div. 16-301,3-Ml 

8 . The Transmission of Radiation, Report No. 2, OEMsr- 

1085, OSRD Special Report 16.4-4, Polaroid Corpora¬ 
tion, November 1943. Div. 16-301.3-MI 

9. Transmission of Radiation, OEMsr-1085, OSRD Special 
Report 16.4-27, Polaroid Corporation. Div. 16-301.3-MI 

10. Infrared Filters and Their Applications, OSRD 4098, 
OEMsr-987, Ohio State University, January 1 to August 
1, 1944. 

11. Manufacture and Inspection of Infrared Filters, OSRD 
4950, OEMsr-987, Ohio State University, Mar. 23, 1945. 

12 . Development of Infrared Plastic Filters — Final, OSRD 
5565, OEMsr-987, Ohio State University, Aug. 31, 1945. 
12a. Ibid., p. 15 and Fig. 14. 

12b. Ibid., p. 10. 

13. Improved Infrared Transmitting Filters, OSRD 5987, 

OEMsr-1085, Service Projects NS-155 and CE-34, Final 
Report 16.4-53, covering the period from August 21, 
1943 to October 31, 1945. Polaroid Corporation, Oct. 31, 
1945. Div. 16-301.3-M3 

13a. Ibid., pp. 65-100. 


Chapter 3 


CASH MAN THALLOUS SULFIDE CELLS a 

1 . Development of Sensitive Thallous Sulfide Photocon- 
ductive Cells for Detection of Near Infrared Radiation, 

“See also bibliographies of equipment using TF cells, in 
Chapters 4 and 5. 


R. J. Cashman, OSRD 1325, OEMsr-235, Report 16.4-6, 
Northwestern University, Mar. 17, 1943. Contains bib¬ 
liography of over 170 references on all types of infra¬ 
red detectors, including 25 references on thallous sulfide 
and related cells. Div. 16-302.11-M2 

2 . Thallous Sulfide Photoconductive Cells, A Summary of 




BIBLIOGRAPHY 


375 


Service Characteristics, W. L. Hole and R. J. Cashman, 
OSRD 1474, NDCrc-185 and OEMsr-235, Report 16.4-7, 
University of Michigan and Northwestern University, 
Mar. 10, 1943. Div. 16-302.11-MI 

3. Development of Stable Thallous Sulfide Photoconduc- 

tive Cells for Detection of Near Infrared Radiation, 
R. J. Cashman, OSRD 5997, OEMsr-235, Service Proj¬ 
ects NS-151, NS-225, and others, Report 16.4-63, cover¬ 
ing the period from December 1941 to October 1945, 
Northwestern University, Oct. 31, 1945. Selected bibli¬ 
ography. Div. 16-302.11-M6 

4. Thallous Sulfide Photoconductive Cell, C. W. Hewlett, 
J. J. Fitz Patrick, and H. T. Wrobel, OSRD 5099, 
OEMsr-1322, Service Project NS-225, Report 16.4-39, 
General Electric Company, Feb. 28, 1945. 

Div. 16-302.11-M3 

5. Development of Methods for Manufacturing the Type 
B Thalofide Cell, Ralph W. Engstrom and Alan M. 
Glover, OSRD 6003, OEMsr-1486, Service Projects 
NS-225, SC-5, and others, Final Report 16.4-69, cover¬ 
ing the period from April 16 to October 31, 1945, Radio 
Corporation of America, Oct. 31, 1945. 

Div. 16-302.11-M7 

6 . Research on Thallous Sulfide Photoconductive Cells, 

Arthur R. von Hippel, and E. S. Rittner, OSRD 4933, 
OEMsr-1036, Service Projects AC-101, SC-5, and 
others, Report 16.4-40, MIT, Laboratory for Insulation 
Research, April 1945. Div. 16-302.11-M5 

7. “Thallous Sulfide Photoconductive Cells,” Arthur R. 
von Hippel and others, MIT; Journal of Chemical 
Physics, Vol. 14, June 1946, Part I, 355; Part II, p. 370. 

8 . The Photoelectric Mechanism of the Thallous Sulfide 
Photoconductive Cell, Arthur R. von Hippel, F. G. 
Chesley, H. S. Denmark, P. B. Ulin, and E. S. Rittner, 
MIT; paper presented at the American Physical So¬ 
ciety meeting at Cambridge, Massachusetts, April 26, 
1946. 

9. A Theoretical Approach to Some Fundamental Proper¬ 

ties of Thallous Sulfide Photoconductive Cells, A. W. 
Ewald, W. L. Hole, and G. E. Uhlenbeck, NDRrc-185, 
Special Report 16.4-26, University of Michigan, Mar. 15, 

1945. Div. 16-302.11-M4 

LEAD SULFIDE PHOTOCONDUCTIVE CELLS 

10. Development of Sensitive Lead Sulfide Photoconductive 

Cells for Detection of Intermediate Infrared Radia¬ 
tion, R. J. Cashman, OSRD 5998, OEMsr-235, Final 
Report 16.4-64, covering the period from January 1944 
to October 1945, Northwestern University, Oct. 31, 
1945. Div. 16-302.12-MI 

11. Exploratory Equipment Using the Lead Sulfide Cell for 

Military Detection Purposes, R. B. Allured, J. G. Black, 
W. L. Hole, and T. R. Kohler, OSRD 6290, NDCrc- 
185, Report 16.4-73, University of Michigan, Oct. 31, 

1945. Div. 16-302.12-M2 

12. Characteristics of Cooled Lead Sulfide Photoconductive 
Cells, Charles L. Oxley, Naval Research Laboratory, 
paper presented at the Optical Society of America meet¬ 
ing at Cleveland, Ohio, Mar. 8, 1946. 


13. Zeiss Lead Sulfide Cells, Field Report TLI-2, Intelli¬ 
gence Branch, Technical Liaison Division, 1945. 

14. German Lichtsprecher 250/130, A. H. Nethercot, Jr., 
and Walter S. Huxford, OSRD 6200, OEMsr-990, Serv¬ 
ice Projects SC-126, SC-128, and SC-129, Report 16.4- 
70, Northwestern University, Oct. 20, 1945. 

Div. 16-303.3-M4 

SILICON PHOTOCONDUCTIVE CELLS 

15. An Investigation of Silicon Photoconductive Cells, G. 
K. Teal, OSRD 5172, OEMsr-1231, Final Report 16.4- 
37, covering period from October 15, 1943 to March 31, 
1944, Bell Telephone Laboratories, Jan. 4, 1945. 

Div. 16-302.14-MI 

16. A New Bridge Photocell Employing a Photoconductive 
Effect in Silicon, Some Properties of High Purity Sili¬ 
con, G. K. Teal, J. R. Fisher, and A. W. Treptow, 
OEMsr-1231, Bell Telephone Laboratories, paper pre¬ 
sented at the American Physical Society meeting at 
Cambridge, Mass., Apr. 26, 1946. 

17. The Velocity of Propagation of the Transmitted Photo¬ 
effect in Silicon Crystals, F. C. Brown, paper presented 
at the American Physical Society meeting at Cam¬ 
bridge, Mass., Apr. 26, 1946. 

SELENIUM ELECTROLYTIC CELLS 

18. Selenium Photocells, Arthur R. von Hippel, OSRD 
1326, OEMsr-561, Progress Report 16.4-2, MIT, Labora¬ 
tory for Insulation Research, January 1943. 

Div. 16-302.13-MI 

19. A New Electrolytic Selenium Photocell, Arthur R. von 
Hippel, J. H. Schulman, and E. S. Rittner, OSRD 1969, 
OEMsr-561, MIT, Laboratory for Insulation Research, 
June 10, 1943. Includes bibliography. 

Div. 16-302.13-M2 

20. “A New Electrolytic Selenium Photocell,” Arthur R. 
von Hippel, J. H. Schulman, and E. S. Rittner, OEMsr- 
561, MIT, Journal of Applied Physics, Yol. 17, No. 4, 
April 1946, p. 215. 

PHOTOTUBES AND PHOTOMULTIPLIERS 

21. Special Phototubes and Circuit Developments, H. Sal¬ 
inger, OSRD 4760, OEMsr-1094, Service Projects NS- 
159, NR-103, and CE-22, Technical Report 16.4-38, 
covering the period from June 15, 1943 to December 31, 

1944, Farnsworth Television and Radio Corporation, 

Jan. 29, 1945. Div. 16-302.21-MI 

22. Special Phototubes and Circuit Developments, H. Sal¬ 
inger, OSRD 5981, OEMsr-1094, Service Projects NS- 
159, NR-103, and CE-22, Final Report 16.4-48, cover¬ 
ing the period from January 1, 1945 to August 31, 1945, 
Farnsworth Television and Radio Corporation, Sept. 26, 

1945 . Div. 16-302.21-M2 

GENERAL 

23. A Photocell Test Set, W. L. Hole and L. N. Holland, 
OSRD 5298, NDCrc-185, Service Projects SC-5, NS-225, 




376 


BIBLIOGRAPHY 


and NS-151, Final Report 16.4-35. covering the period 
from November 1, 1943 to July 19, 1944, University of 
4 Michigan, Apr. 4, 1945. Div. 16-302.1-MI 

24. Operating Instructions for Photocell Test Set, L. N. 
Holland. Special Report 16.4-6, NDCrc-185, University 
of Michigan, Apr. 25, 1944. 

25. Operating Instructions for Microflux Source, Model 3, 
W. L. Hole, NDCrc-185, Special Report 16.4-9. Uni¬ 
versity of Michigan, June 9, 1944. 

26. New Photoconductive Cells, R. J. Cashman, Northwest¬ 
ern University, paper presented at the Optical Society 
of America meeting at Cleveland, Ohio, on Mar. 8, 
1946. 

27. Some Technical Developments in Light Wave Com¬ 
munication, John M. Fluke and Noel E. Porter, Section 
660-E Ships, paper presented at the Institute of Radio 
Engineers meeting in New York Cit\’, Jan. 23-26. 
1946. 

28. “On the Theory of the Brownian Motion II,” G. E. 
Uhlenbeck and Ming Chen Wang. Reviews of Modem 
Physics, Vol. 17, Nos. 2 and 3, April to July, 1945, p. 323. 


29. “Recent Contributions to the Theory of Random Func¬ 
tions,” N. Levinson, W. Pitts and W. F. Whitmore, 
Science, Vol. 103, No. 2670, Mar. 1, 1946, p. 283. 

30. Infrared Voice and Code Communication Systems, 
Navy Type E, B. J. Spence, OSRD 5999, OEMsr-990, 
Service Project NS-159, Final Report 16.4-65, covering 
the period from May 1943 to October 1945, North¬ 
western University, Oct. 31, 1945. Div. 16-303.2-M5 
30a. Ibid, Appendix E, Theoretical Aspects of Optical 

Communication Systems, Hartland S. Snyder and 
John R. Platt, also issued as OSRD 6289. Report 
16.4-72, Oct. 31, 1945. Div. 16-303.1-MI 

30b. Ibid., Appendix G, Noise in Photodetectors, Hart- 
land S. Snyder. 

31. Design Considerations for Wide-Angle Multipliers, H. 
Salinger, Research Memorandum 49, Farnsworth Tele¬ 
vision and Radio Corporation, Jan. 18, 1944. 

32. The Modern Theory of Solids, Frederick Seitz, McGraw- 
Hill Book Co., New Y T ork, 1940. 

33. Electronic Processes in Ionic Crystals, N. F. Mott and 
R. W. Gurney, Oxford University Press, 1940. 


Chapter 4 


FOREIGN SYSTEMS AND AMERICAN SYSTEMS 
NOT DEVELOPED UNDER NDRC 

1. “Neue Apparate fiir Lichttelephonie,” H. Thirring. Phys- 
ikalische Zeitschrift, Vol. 21, No. 3, Feb. 1, 1920. p. 67. 

2. “Suggested Methods of Secret Signalling,” E. Beckmann 
and P. Knipping, Sitzungsberichte der Preussischen 
Akademie der Wissenschaften zu Berlin, Vol. 25, 1920, 
p. 443; Science Abstracts 24A, No. 85S, 1921. 

3. “Infrared Telegraphy and Telephony,” T. W. Case, Jour¬ 
nal of the Optical Society of America, Vol. 6, No. 4, 
June 1922, p. 398. 

4. “American Physicists at War,” I. Bernard Cohen, 
Amencan Journal of Physics, Vol. 13, No. 5, October 
1945, p. 333. Includes nonclassified bibliography. 

5. “German ‘Speech-on-Light’ Signal System,” D. Gifford, 
Radio, November 1943, p. 32. 

6 . Report on German 1R Speech Communication Appa¬ 

ratus, Li-80, C. V. Kent and W. L. Hole, NDCrc-185, 
Special Report 16.4-1, University of Michigan, Sept. 11, 
1943. Div. 16-303,3-Ml 

7. German Lichtsprecher 250 (made by Zeiss 1942), Anal¬ 
ysis of Transmitter Principles, Harvey E. White, 
OEMsr-1073, Special Report 16.4-46, University of 
California, 1945. 

8 . German Lichtsprecher 250/130, A. H. Nethercot, Jr. and 
Walter S. Huxford, OSRD 6200. OEMsr-990, Service 
Projects SC-126, SC-128, and SC-129, Report 16.4-70, 
Northwestern University, Oct. 20, 1945. 

Div. 16-303.3-M4 

9. Japanese Light-Beam Telephone, A. H. Nethercot, Jr. 
and Walter S. Huxford, OSRD 5763, Service Projects 
SC-126, SC-128, and SC-129, Report 16.4—46. North¬ 
western University, Aug. 1, 1945. Div. 16-303.3-M2 


10. Instructions for RCA Light Communication System, 
Type R-2, Contract W-3434-sc-69, RCA Victor Divi¬ 
sion of Radio Corporation of America with Army Sig¬ 
nal Corps, 1944. 

11. Optiphone AN/TVC-1 (A '0-1), War Department Tech¬ 
nical Manual, TM 11-395, Jan. 27, 1945. 

TYPE W 

12 . A Portable Hand-Held Infrared Optical Telephone, 

Harvey E. White, Alfred Einarsson, and Charles G. 
Miller, OSRD 6002. OEMsr-1073, Service Projects AC- 
226.03 and NS-371, Final Report 16.4-68, University of 
California, Nov. 30, 1945. Div. 16-303.1-M2 

TYPE E 

(formerly designated as Type N) 

13. Receiving Unit for Voice Transmission on Light Beam , 

Everett W. Lothrop, Jr. and Hartland S. Snj’der, OSRD 
3122. OEMsr-990, Report 16.4-16, Northwestern Uni¬ 
versity, Jan. 1, 1944. Div. 16-303.2-MI 

14. Modulator for 100-Watt Concentrated Arc, B. J. Spence, 

OSRD 3123, OEMsr-990, Report 16.4-17, Northwestern 
University, Jan. 1, 1944. Div. 16-301.12-MI 

15. Transmitter and Receiver Optics: Point Source-Voice 
System, John R. Platt, OEMsr-990, Special Report 16.4- 
14, Northwestern University, June 22, 1944. 

16. Report on Transmission Tests of Type N Equipment, 
Walter S. Huxford, OEMsr-990, Special Report 16.4-19, 
Northwestern University, Dec. 14, 1944. 

Div. 16-303.2-M2 

17. Report on Tests of Type N Equipment at Ft. Miles, 
Lewes, Delaware, 14-18 November 1944, M. J. Stola- 



BIBLIOGRAPHY 


377 


roff. File S-S67-(12) (660d), Serial 00227, Section 660d, 
BuShips, Dec. 5, 1944. Includes other BuShips refer¬ 
ences. 

18. Infrared Voice and Code Communication Systems. 
Navy Type E, B. J. Spence, OSRD 5999, OEMsr-990, 
Service Project NS-159, Final Report 16.4-65, North¬ 
western University, Oct. 31, 1945. Div. 16-303.2-M5 
18a. Ibid., p. 73 ff, pp. 106-7. 

18b. Ibid., pp. 142-3. 

19. Test No. 1 on US/E-2 Nancy Equipment Manufactured 

by Belmont Radio Corporation, OEMsr-990, Sendee 
Project XS-159. Special Report 16.4-41, Northwestern 
University, Sept. 17, 1945. Div. 16-303.2-M4 

20. Test No. 1 on US/E-1 Nancy Equipment Manufac¬ 
tured by Cover-Dual Signal Syste?ns, Inc., OEMsr-990, 
Sendee Project NS-159, Special Report 16.4-42, North¬ 
western University, Nov. 15, 1945. Div. 16-303.2-M7 

21. Report on the Status of Electroacoustic Performance 
Qualifications for the Differential Microphone Used 
with Type E Equipment, R. L. Osborn, OEMsr-990, 
Special Report 16.4-43, Northwestern University, Oct. 
24, 1945. 

22. Detection of Navy Type E Source by German Seekund, 
Walter S. Huxford, OEMsr-990, Service Projects SC- 
126, SC-128, and SC-129, Special Report 16.4-45, North¬ 
western University, Dec. 27, 1945. Div. 16-310.33-MI 

23. Theoretical Aspects of Optical Communication Systems, 

Hartland S. Snyder and John R. Platt, OSRD 6289, 
OEMsr-990, Sendee Project NS-159, Northwestern 
University, Oct. 31, 1945. Div. 16-303.1-MI 

AIRCRAFT SYSTEMS 

24. Infrared Voice Communication Systems for Aircraft, 
Everett W. Lothrop, Jr., John R. Platt, and Wallace 
R. Wilson, OSRD 6000, OEMsr-900, Service Projects 
AC-226.03 and AC-226.04, Final Report 16.4-66, North¬ 
western University, Oct. 31, 1945. Div. 16-303.2-M6 

CARRIER-WAVE SYSTEMS 

25. Infrared Communication Systems, F. Srnardo, OSRD 
5373, OEMsr-1460, Final Report 16.4-45, V-M Corpora¬ 
tion, Benton Harbor, Michigan, July 19, 1945. 

Div. 16-303.2-M3 

26. Infrared Communication System of Captain Guy Tou- 

vet, B. J. Spence, OSRD 6001, OEMsr-1391, Service 
Project NS-243, Report 16.4-67, Northwestern Univer¬ 
sity, Oct. 10, 1945. Div. 16-303.3-M3 

PHOTOELASTIC SHUTTER 

27. Development of a Photoelastic Shutter for Modulating 
Infrared Light at Audiofrequencies, Hans Mueller, 
OSRD 1296, OEMsr-576, Research Project DIC-60S5, 
Report 16.4-1, MIT, Jan. 1, 1943. Div. 16-301.2-MI 

28. Memorandum on the Polarization Optics of the Photo¬ 

elastic Shutter, Hans Mueller, OSRD 3171, OEMsr-576, 
Research Project DIC-60S5, Report 16.4-18, MIT, Nov. 
15, 1943. Div. 16-301.2-M2 

29. Development of a Photoelastic Shutter for Voice-Com¬ 
munication over a Beam of Infrared Light, Hans 


Mueller and others, OEMsr-576, Special Report 16.4- 
47, MIT, Jan. 6, 1946. 

29a. Ibid., p. 6. 

GENERAL 

30. Some Technical Development in Light Wave Com¬ 
munication, John M. Fluke and Noel E. Porter, Sec¬ 
tion 660E, BuShips, paper given before Institute of 
Radio Engineers meeting in New Y"ork City. Jan. 23-26, 
1946. 

31. A Method for the Computation of Threshold Range as 
Affected by Instrumental Thresholds and Atmospheric 
Attenuation, W. L. Hole, OSRD 3345, NDCrc-185, Re¬ 
port 16.4-22, including as appendix Report 16.4-21, 
University of Michigan, Feb. 19, 1944. 

Div. 16-320.2-MI 

32. Preliminary Report No. 1 on Atmospheric Attenuation 
of Infrared Radiation, John Strong, OEMsr-60, Special 
Report 16.4-31, Harvard University, May 29, 1945. 

Div. 16-320.2-M2 

33. Atmospheric Attenuation of Infrared Radiations, John 
D. Strong, OSRD 5986, OEMsr-60, Final Report 16.4- 
52, Harvard University, Nov. 30, 1945. 

Div. 16-320.2-M4 

34. “Mean Absorption and Equivalent Absorption Coeffi¬ 
cient of a Band Spectrum,” Walter M. Elsasser, The 
Physical Review, Vol. 54, No. 2, July 15, 1938, p. 126. 

35. “The Absorption Laws for Gases in the Infra-Red,” 
J. Rud Nielsen, Y. Thornton, and E. Brock Dale, 
Reviews of Modem Physics, Vol. 16, Nos. 3 and 4, 
July to October 1944, p. 307. 

36. ‘‘On a New Method of Measuring the Mean Height of 
the Ozone in the Atmosphere,” John Strong, Journal of 
the Franklin Institute, Vol. 231, No. 2, February 1941, 
p. 138. 

37. “Optics of Atmospheric Haze,” E. 0. Hulburt, Journal 
of the Optical Society of America, Vol. 31, No. 7, 
July 1941, p. 472. 

38. “On the Relation between Visibility and the Constitu¬ 
tion of Clouds and Fog,” H. G. Houghton, Journal of 
the Aeronautical Sciences, Vol. 6, 1939, p. 408 

39. The Development of Apparatus for the Detection of 
Night Bombing Planes by Near Infrared Radiation, 
NDCrc-185, Engineering Research Project M-341, Re¬ 
port D-3, 244, University of Michigan, June 1, 1942. 

Div. 16-304.2-MI 

40. A Near Infrared System for Recognition and Ship-to- 

Ship Communication, Type D-2, J. G. Black, P. H. 
Geiger, and A. F. Fairbanks, OSRD 5993, NDCrc-185, 
Service Project NS-151, Final Report 16.4-59, Univer¬ 
sity of Michigan, Oct. 31, 1945. Div. 16-304.1-M2 

41. On the Articulation Efficiency of Bands of Speech in 
Noise, J. P. Egan and F. M. Wiener, OSRD 4872, Har¬ 
vard University, May 1, 1945. 

42. Section 764 Bimonthly Summary Reports, 1943-1946. 

43. Notes on Generalized Photometry with Particular Ap¬ 
plication to the Near Infrared (revised schematic ver¬ 
sion), G. A. Van Lear, Jr., OSRD 1585, NDCrc-185, 
Report 16.4-10, University of Michigan, June 16, 1943. 

Div. 16-302.2-M3 





378 


BIBLIOGRAPHY 


Chapter 5 


TYPE D 

1. Report on an Infrared Recognition Device Developed 
at the University of Michigan and Demonstrated at 
Norfolk, Virginia, on February 12, 19^3, P. H. Geiger, 
J. G. Black, and E. F. Barker, OSRD 1654, NDCrc-185, 
Report 16.4-12, University of Michigan, Apr. 1, 1943. 

Div. 16-304.2-M2 

2. An Infrared Radiation System for Recognition and 
Ship to Ship Communication, J. G. Black and P. H. 
Geiger, OSRD 3546, NDCrc-185, Report 16.4-23, Uni¬ 
versity of Michigan, Mar. 1, 1944. Div. 16-304.1-MI 

3. A Near Infrared System for Recognition and Ship to 

Ship Communication, Type D-2, J. G. Black, P. H. 
Geiger, and A. F. Fairbanks, OSRD 5993, NDCrc-185, 
Service Project NS-151, Final Report 16.4-59. Univer¬ 
sity of Michigan, Oct. 31, 1945. Div. 16-304.1-M2 

4. Report on the Tests Made on the First Emerson Type 
D Receiver, P. H. Geiger, NDCrc-185, Special Report 
16.4-7, University of Michigan, May 29, 1944. 

5. Report on the Tests Made on the First Crouse-Hinds 
Type D Beacons, J. G. Black and Norman E. Barnett, 
NDCrc-185, Special Report 16.4-8, University of Michi¬ 
gan, June 1, 1944. 

6 . Operational Tests of Type D Equipment, Lawrence 
Dunkelman and Joseph Ballam, Report IV (33M) Elec¬ 
trical Section (660) BuShips, March 1944. 

6 a. Ibid., p. 14. 

6 b. Ibid., p. 34. 

7. Preliminary Instructions for U. S. Navy D-l Nancy 
Equipment, Navy Contract NObs 16077, RCA Manu¬ 
facturing Company, Camden, N. J., 1944. 

8 . Some Technical Developments in Light-Wave Com¬ 
munications, John M. Fluke and Noel E. Porter, Bu¬ 
Ships Electrical Section, Paper given before New York 


meeting of Institute of Radio Engineers, Jan. 23-26, 
1946, Proceedings of the Institute of Radio Engineers, 
November 1946. 

PLANE-TO-PLANE RECOGNITION SYSTEM 

9. Plane-to-Plane Recognition, P. H. Geiger and J. G. 
Black, OSRD 5994, NDCrc-185, Service Projects NA- 
194 and AC-101, Final Report 16.4-60. University of 
Michigan, Nov. 1, 1945. Div. 16-304.2-M3 

RETRODIRECTIVE TARGET LOCATOR 

10. Preliminary Development and Tests of a Retrodirective 
Locator for the Visible and Near Infrared (including 
as appendix: A Method for the Computation of Thresh¬ 
old Range as Affected by Instrumental Thresholds and 
Atmospheric Attenuation, also issued as Report 16.4-21), 
W. L. Hole, OSRD 3343. NDCrc-186, Report 16.4-22, 
University of Michigan, Feb. 19. 1944. 

Div. 16-320 2-MI 

JAPIR 

11 . Japir Detection Equipment, P. H. Geiger and J. G. 

Black, OSRD 5995, NDCrc-185, Service Project NA- 
191, Final Report 16.4-61, University of Michigan, Oct. 
17, 1945. Div. 16-302,3-Ml 

12. Operatory Manual for NA 191 Equipment, P. H. Geiger, 
J. G. Black and T. R. Kohler, NDCrc-185, Special Re¬ 
port 16.4-32, University of Michigan, March 1945. 

Div. 16-302.3-M2 

13. JAPIR NAN Remote Indicating Receiver Installed 
Test for Satisfactory Operation, Project TED PTR- 
31648.0, Radio Test at Naval Air Station, Patuxent 
River, Md., Feb. 8, 1945. 


Chapter 6 


1. Night Surveying and Signalling by Infrared [IRRAD], 
E. F. Kingsbury, OSRD 3211, OEMsr-766, Western 
Electric Company, Dec. 1, 1943. 

2. Night Surveying by Infrared [IRRAD], R. C. Mathes, 
OSRD 4850, OEMsr-1267, Service Projects CE-22 and 
NR-103, Progress Report 16.4-36, BTL, Dec. 27, 1944. 

Div. 16-305.1-MI 

3. Night Surveying by Infrared [IRRAD], R. C. Mathes, 
OSRD 5982, OEMsr-1267, Service Projects CE-22 and 
NR-103, Final Report 16.4-49, BTL, July 27, 1945. 

Div. 16-305.1-M2 


4. IRRAD Equipment for Night Surveying, War Depart¬ 
ment Report 937, The Engineer Board, Corps of 
Engineers, U. S. Army, Fort Belvoir, Va., May 23, 
1945. 

5. An Infrared Range and Direction Apparatus for Dif¬ 
fusely Reflecting Targets (Diffuse IRRAD), W. L. Hole, 
W. W. McCormick, and L. N. Holland, NDCrc-185, 
Service Projects CE-22 and NR-103. Final Special Re¬ 
port 16.4-21, University of Michigan, Sept. 17, 1945. 

Div. 16-305.2-M3 


Chapter 7 


1. Development of an Infrared Glider Position Indicator, 
G. A. Van Lear, Jr. and L. N. Holland, OSRD 4752, 
NDCrc-185, Final Report 16.4-31, University of Michi¬ 
gan, Jan. 11, 1945. Div. 16-320.1-M2 


2. Cloud Attenuation Studies, Mount Washington, New 
Hampshire, G. A. Van Lear, Jr., OSRD 6201, NDCrc- 
185, Service Project AC-56, Final Report 16.4-71, Uni¬ 
versity of Michigan, Oct. 25, 1945. Div. 16-320.2-M3 



BIBLIOGRAPHY 


379 


3. Visibility in Meteorology, W. E. Knowles Middleton, 
University of Toronto Press, 1941, p. 51. 

4. “Optics of Atmospheric Haze,” E. O. Hulburt, Journal 
of the Optical Society of America, Vol. 31, 1941, 
p. 467. 

5. “Transmission of Light in the Atmosphere with Appli¬ 


cations to Aviation,” H. G. Houghton, Journal of Aero¬ 
nautical Science, Yol. 9, 1942, p. 103. 

6 . Notes on Generalized Photometry with Particular Ap¬ 
plication to the Near Infrared, G. A. Van Lear, Jr., 
OSRD 1585, NDCrc-185, Report 16.4-10, University of 
Michigan, June 16, 1943. Div. 16-302.2-M3 


Chapter 8 


1 . Smithsonian Miscellaneous Collections, Frederick E. 
Fowle, Smithsonian Institution of Washington. D. C., 
Vol. 68, No. 8, 1917. 

2 . Comparative Testing of Thermal Detectors, Harald N. 

Nielsen, OSRD 5992, OEMsr—1168, Service Project 
AN-6, Report 16.4-58, Ohio State University, Oct. 31, 
1945. Div. 16-310.2-M4 

2a. Ibid., p. 70 ff. 

3. Improved Far Infrared Receivers and Associated Op¬ 

tics, Louis Harris, OSRD 5299, OEMsr-1147, Service 
Projects AC-34, NS-157, and NS-161, Final Report 
16.4-41, MIT, July 10, 1945. Div. 16-310.21-M2 

4. The Frequency Characteristics of Bolometers, Charles 
B. Aiken, OEMsr-921, Service Project AC-36, Electro- 
Mechanical Research, Inc., Nov. 17, 1944. 

Div. 5-332-M2 

5. A Radiation Source for Testing Thermistor Bolometers, 
Cases 23244 and 25717, N. Van Roosbrocsk, Report 
MM-45-2980-4, BTL, Aug. 30, 1945. 

6 . Test Set and Test Procedure for Thermistor Bolome¬ 
ters, Cases 23244, 25717 and 25787, J. B. Johnson, Report 
MM-45-2930-15, BTL, Aug. 22, 1945. 

7. 15-Cycle Bolometer Amplifier, per ES 832 446, Case 
23244, H. R. Moore, BTL, Oct. 15, 1943. 

8 . “Thermocouples for the Measurement of Small Inten¬ 
sities of Radiations,” Louis Harris, The Physical Re¬ 
view, Vol. 45, 1934, p. 635. 

9. Sputtered and Evaporated Thermopiles, and Studies of 


Their Films of Plastic Materials, Louis Harris, OSRD 
4415, MIT, Mar. 31, 1943. 

10 . Construction of Evaporated Thermopiles, A. Ph.D. Dis¬ 
sertation, A. Stockfleth, MIT, November, 1942. 

11. Bolometer and Infrared Receivei Components, Robert 
Mack, John D. Strong, and Noel Jamison, OSRD 5983, 
OEMsr-60, Service Projects NS-121, N-108, and others, 
Report 16.4-47, Harvard University, Dec. 31, 1945. 

Div. 16-310.223-M2 

12. Development and Operating Characteristics of Thermi¬ 
stor Bolometers and Their Application in Infrared De¬ 
vices, J. A. Becker, Walter H. Brattain, and others, 
OSRD 5991, OEMsr-636, Service Projects NS-161 and 
AC-225.01, Final Report 16.4-57, BTL, Oct. 31, 1945. 

Div. 16-310.221-M7 

13. The Modern Theory of Solids, Frederick Seitz, McGraw- 
Hill Book Co., 1940, pp. 65, 189. 

14. The Optical Properties of Thermistor Material in the 

Infrared, A. H. Pfund, Special Report 16.4-22, Dec. 26, 
1944. Div. 16-310.221-M4 

15. Useful Equations in the Design of Thermistors, J. A. 
Becker and W. H. Brattain, BTL, Jan. 21, 1944. 

16. Heat Detection (Engineering Report), Marcel Golay, 
Supplement ESL-52, Ohio State Research Foundation, 
June 1, 1943. 

17. Thermistor Bolometer and Amplifier for Infrared Spec¬ 
trometers, J. A. Becker and H. Cristensen, OSRD 3786, 
OEMsr-1098, Final Report 16.4-20, BTL, June 2, 1944. 

Div. 16-310.221-MI 


Chapter 9 


1 . Transmission of Infrared Radiation through Atmo¬ 

spheric Media, Harald H. Nielsen and Ely E. Bell, 
OSRD 3799, OEMsr-1168, Progress Report 16.4-27, Ohio 
State University, May 31, 1944. Div. 16-310.1-M2 

2 . Thermal Radiation from Targets and Backgrounds, 

John D. Strong, OSRD 5372, OEMsr-60, Service Project 
NS-163, Technical Report 16.4-44, Harvard University, 
Mar. 30, 1945. Div. 16-310.1-M3 

3. “Atmospheric Absorption of Infrared Solar Radiation at 
the Lowell Observatory,” I. Arthur Adel, Astrophysical 
Journal, Vol. 89, 1939, p. 1. 

4. “Study of Atmospheric Ahgorption and Emission in the 
Infrared Spectrum,” John D. Strong, Journal of the 
Franklin Institute, Vol. 232, July 1941, p. 1. 

5. Atmospheric Attenuation of Infrared Radiations, John 

D. Strong, OSRD 5986, OEMsr-60, Service Project 
AN-32, Final Report 16.4-52, Harvard University, Nov. 
30, 1945. Div. 16-320.2-M4 


6 . Infrared Search and Indicator Units for Drone Control 
Models A and B of the Type L System, H. R. Moore, 
OSRD 5989, OEMsr-636, Service Project NA-172, 
Final Report 16.4-55, BTL, Oct. 31, 1945. 

Div. 16-310.221-M6 

7. “The Polarization of Light at Sea,” E. 0. Hulburt, 
Journal of the Optical Society of America, Vol. 24, 1934, 
p. 35. 

8 . Oceanography for Meteorologists, H. U. Sverdrup, Pren¬ 
tice-Hall, Inc., New York, 1942, p. 225, Fig. 63. 

9. The Portable Infrared Detector, J. A. Becker, W. H. 

Brattain, L. M. Ugenfritz, H. R. Moore, and L. F. Stach- 
ler, OSRD, OEMsr-636, Final Report 16.4-28, BTL, 
May 29, 1944. Div. 16-310.311-MI 

10. Development of the SND-1, W. H. Brattain and N. G. 
Wade, OSRD 5988, OEMsr-636, Service Projects CE-37 
and AC-225.02, Final Report 16.4-54, BTL, Oct. 15, 
1945. Div. 16-310.312-M3 






380 


BIBLIOGRAPHY 


11. Report on Tests with SND for the Detection of Tanks 
at Fort Belvoir, Virginia, on January 14, 15 and 16, 
1945, OEMsr-636, Special Report 16.4-23, BTL, Jan. 23, 
1945. 

12. Development of the Strip Map Recorder for Target 

Survey, L. M. Ilgenfritz, R. W. Ketledge, P. B. Murphy, 
and J. T. Scott, OSRD 5990, OEMsr-636, BTL, Oct. 31, 
1945. Div. 16-310.313-M2 

13. Ground Tests of Strip Map Recorder at Fort Knox, 
July 9-11, 1945, L. M. Ilgenfritz, OEMsr-636, Special 
Report 16.4-36, BTL, Aug. 4, 1945. Div. 16-310.313-MI 

14. Airborne Tank Detection Tests with Strip Map Re¬ 
corder, L. M. Ilgenfritz, OEMsr-636, Special Report 
16.4-39, BTL, Aug. 30, 1945. 

15. An Assessment of a Far Infrared Bombsight with An¬ 
gular Rate Release, S. Darlington, R. W. Buntenbach, 
and others, OEMsr-636, Service Project NO-258, Spe¬ 
cial Report 16.4-18, BTL, Dec. 7, 1944. 

Div. 16-320.3-MI 

16. The Portable Ship Detector, PSD, OSRD 5984, OEMsr- 
60, Harvard University, Dec. 31, 1945. 

Div. 16-310.321-MI 

17. Stabilized Ship Detector, SSD, OSRD 5985, OEMsr-60, 
Harvard University, Dec. 31, 1945. 

Div. 16-310.332-MI 

18. Bolometers and Optical Components of Infrared Re¬ 
ceivers for Military Applications, OSRD 5983, OEMsr- 
60, Harvard University, Dec. 31, 1945. 

Div. 16-310.223-M2 

19. Infrared Radiation from Targets LCT and LCI Em¬ 
ployed in Trials of Ship Detectors at Cape Henlopen, 
J. A. Sanderson, Report H-2249, Naval Research Labor¬ 
atory, Feb. 28, 1944. 

20. Special Report on SSD Tests on the Marnell, Octo¬ 


ber 22, 1944 to November 3, 1944 > N. C. Jamison, 
OEMsr-60, Special Report 16.4-25, Harvard University, 
Feb. 12, 1945. Div. 16-310.322-M2 

21. Exploratory Equipment Using the Lead-Sulphide Cell 
for Military Detection Purposes, R. B. Allured, J. G. 
Black, W. L. Hole and T. R. Kohler, OSRD 6290, 
NDCrc-185, University of Michigan, Nov. 1, 1945. 

Div. 16-302.12-M2 

22. Development of Sensitive Lead Sulphide Photocondwc- 
tive Cells, R. J. Cashman, OSRD 5998, OEMsr-235, 
Northwestern University, Oct. 31, 1945. 

Div. 16-302.12-MI 

23. A Photocell Test Set, L. N. Holland and W. L. Hole, 

OSRD 5298, NDCrc-185, Service Projects SC-5, NS- 
225, and NS-151, Final Report 16.4-35, University of 
Michigan, Apr. 4, 1945. Div. 16-302.1-MI 

24. Infrared Rangefinder, John D. Strong, OSRD 5192, 
OEMsr-60, Service Project NO-183, Technical Report 
16.4-42, Harvard University, Feb. 15, 1945. 

Div. 16-305.2-M2 

25. Thermistor Bolometer and Amplifiers for Infrared 

Spectrometers, J. A. Becker and H. Christensen, OSRD 
3786, OEMsr-1098, Final Report 16.4-20, BTL, June 2, 
1944. Div. 16-310.221-MI 

26. An Investigation of the Comparative Merits of the 

Thermistor — a-c Amplifier System and the Thermo- 
couple-Galvonometer System, L. W. Gildart, OEMsr- 
1132, Special Report 16.4-24, University of Michigan, 
Jan. 21, 1944. Div. 16-310.23-M2 

27. Memorandum on Test of Thermistor Bolometers as Re¬ 

ceivers for National Technical Laboratories Routine 
Infrared Spectrophotometer, R. R. Brattain and O. 
Beeck, Shell Development Company, Emeryville, Cali¬ 
fornia, Jan. 4-13, 1944. Div. 6-310.23-MI 



OSRD APPOINTEES 


division 16 
Chief 

George R. Harrison 


Deputy Chiefs 

Paul E. Klopsteg 
Richard C. Lord 


Consultants 

Herbert E. Ives 
F. E. Tuttle 


Technical Aides 

Richard C. Lord 
H. K. Stephenson 


H. R. Clark 
J. S. Coleman 


M embers 

0. S. Duffendack 
Theodore Dunham, Jr. 

E. A. Eckhardt 
Harvey Fletcher 
W. E. Forsythe 


Arthur C. Hardy 
Herbert E. Ives 
Paul E. Klopsteg 
Brian O’Brien 
F. E. Tuttle 


section 16.1 
Chief 

Theodore Dunham, Jr. 


Consultants 

G. W. Morey 
F. L. Jones 

H. F. Mark 
H. F. Weaver 


Technical Aides 

Lillian Elveback S. W. McCuskey 

H. F. Weaver 


Members 


Ira S. Bowen 
W. V. Houston 


F. E. Wright 


R. R. McMath 
G. W. Morey 


OSRD APPOINTEES ( Continued ) 


W. R. Brode 


W. E. Forsythe 


S. Q. Duntley 


Edwin G. Boring 


section 16.2 


Chief 

Brian O’Brien 


Consultants 


Technical Aide 

Chas. E. Waring 


Members 

Harvey E. White 


section 16.3 
Chief 

Arthur C. Hardy 

Consultants 

Lewis Knudson 
Parry H. Moon 
Edward R. Schwarz 

Technical Aides 

Ernest T. Larson 

Members 

F. C. Whitmore 


section 16.4 
Chief 

0. S. Duffendack 


Consultants 

W. L. Enfield 
H. G. Houghton, Jr. 
W. H. Radford 


V. K. Zworykin 


Julian H. Webb 


Arthur W. Kenney 


L. A. Jones 


382 





OSRD APPOINTEES ( Continued) 


H. S. Bull 


Alan C. Bemis 
Saul Dushman 


William Herriott 


D. W. Bronk 
A. C. Hardy 
Theodore Matson 
A. H. Pfund 


Technical Aides 

Winston L. Hole 

James S. Owens 

Members 

H. G. Houghton, Jr. 
George A. Morton 

section 16.5 
Chiefs 

W. E. Forsythe 
Herbert E. Ives 

Deputy Chiefs 

W. E. Forsythe 
Brian O’Brien 

Consultants 

E. Q. Adams 

A. C. Downes 

Technical Aides 

John T. Remey 

Val E. Sauerwein 


Members 

W. B. Rayton 
A. B. Simmons 
G. F. A. Stutz 
Harvey E. White 


V. K. Zworykin 


CONTRACT NUMBERS, CONTRACTORS, AND SUBJECTS OF CONTRACTS 


Contract 

Number Name and Address of Contractor Subject 


NDCrc-180 

NDCrc-185 

OEMsr-60 

OEMsr-235 

OEMsr-561 

OEMsr-576 

OEMsr-610 

OEMsr-636 

OEM si-984 

OEMsr-987 

OEMsr-990 

OEM.sr-1036 

OEMsr-1073 

OEMsr-1085 

OEM.sr-1094 

OEMsr-1098 

OEMsr-1132 

OEMsr-1147 

OEMsr-1168 

OEMsr-1231 

OEMsr-1267 

OEMsr-1322 

OEMsr-1391 

OEMsr-1460 

OEMsr-1486 


Massachusetts Institute of Technology 
Cambridge, Massachusetts 
University of Michigan 
Ann Arbor, Michigan 
Harvard University 
Cambridge, Massachusetts 
Northwestern University 
Evanston, Illinois 

Massachusetts Institute of Technology 
Cambridge, Massachusetts 
Massachusetts Institute of Technology 
Cambridge, Massachusetts 
Johns Hopkins University 
Baltimore, Marjdand 
Bell Telephone Laboratories 
Murray Hill, New Jersey 
Western Union Telegraph Co. 

Water Mill, Long Island, New York 
Ohio State University 
Columbus, Ohio 
Northwestern University 
Evanston, Illinois 

Massachusetts Institute of Technology 
Cambridge, Massachusetts 
University of California 
Berkeley, California 
Polaroid Corporation 
Cambridge, Massachusetts 
Farnsworth Television and Radio Cor¬ 
poration 

Fort Wayne, Indiana 
Bell Telephone Laboratories 
New York, New York 
University of Michigan 
Ann Arbor, Michigan 
Massachusetts Institute of Technology 
Cambridge, Massachusetts 
Ohio State University 
Columbus, Ohio 
Bell Telephone Laboratories 
120 Broadway 
New York, New York 
Bell Telephone Laboratories 
120 Broadway 
New York, New York 
General Electric Company 
West Lynn, Massachusetts 
Northwestern University 
Evanston, Illinois 
V-M Corporation 
Benton Harbor, Michigan 
Radio Corporation of America 
Lancaster, Pennsylvania 


Far infrared homing bomb control 
Near infrared detection and signaling 
Far infrared receiver and detector development 
Thalofide and other photoconductive cells 
Photocell development 

Infrared communication with photoelastic shutter 
Infrared optics 

Development of thermistor bolometer and far infrared 
detection equipments 
Electrically modulated arc lamp 

Infrared transmitting filter development 

Infrared communication systems with electrically modu¬ 
lated lamps 
Photocell development 

Mechanically modulated infrared communication system 
Infrared transmitting filters 
Electron multiplier development 

Thermistor bolometers and circuits for spectrometers 
Testing of bolometers and control circuits in spectrometers 
Far infrared receivers and associated optics 
Comparative testing of thermal detectors 
Silicon photoconductive cells 

Infrared range and direction equipment (Irrad) 

Thalofide cell manufacturing development 

Modulated infrared beam communication transmitter and 
receiver 

Infrared communication system 
Thalofide cell manufacturing development 


384 





SERVICE PROJECT NUMBERS 


The projects listed below were transmitted to the Office of the 
Executive Secretary, OSRD, from the War or Navy Department 
through either the War Department Liaison Officer for NDRC or 
the Office of Research and Inventions (formerly the Coordinator of 
Research and Development), Navy Department. 


Service Project Number Title 


AC-56 
AC-63 
AC-101 
AC-225 
AC-225.01 
(supersedes AC-34) 
AC-225.02 
(supersedes AC-87; 
AC-113) 

AC-226 
AC-226.01 
(supersedes AC-63) 
AC-226.03 

(supersedes SC-117) 
AC-226.04 

(supersedes SC-117) 
AN-6 
AN-32 
CE-22 
CE-34 
CE-37 
OD-147 
OD-173 
SC-5 
SC-117 
SC-126 
SC-126.3 
SC-126.4 
SC-127 
SC-128 
SC-128.1 
SC-128.2 
SC-129 
SC-129.1 


N-108 

NA-172 

NA-191 

NA-194 

NO-183 

NO-258 

NR-103 

NS-121 

NS-151 

NS-155 

NS-157 

NS-159 


Army Projects 

Infrared Device for Determining the Position of a Glider with Respect to a Tow Plane 

Protective Coatings for Mirrors and Prisms 

NIR Airplane System 

Infared and Light Target Seekers 

Research and Development of Thermistors and Associated Control Circuits for Use in Heat 
Responsive, Target Seeking, Controllable Bombs 
Research and Development of Thermal Receiving Sj r stems for Installation in Aircraft for the 
Purpose of Conducting Ground Surveys 

Infrared Aids 

Study of Infrared Sources and Receivers Including Associated Basic Research on Optics, etc. 
Investigation and Development of Infrared Troop Carrier Aids 

Investigation and Development of Infrared Aids for Communication between Bombers in 
Formation 

Comparative Testing of Thermal Detectors 

Investigation of Atmospheric Transmissivity Throughout the Infrared Spectrum 
Development of Irrad Equipment 

Image Forming Infrared Equipment (including IR Filters) 

Applications of Far Infrared Equipment to Ground Forces 
Lights for Photography of High Velocity Projectiles 
Adaptation of Type 10 Lamp for De Brie Camera 
Near Infrared Radiation 

NIR Communication for Night Formation Flying 
Testing of Enemy Near Infrared Signal Equipment 
Lichtsprecher 250 
Lichtsprecher 80 

Testing of Enemy Far Infrared Signal Equipment 

Testing of Enemy Signal Infrared Filters 

Filters for Lichtsprecher 250 

Filters for Lichtsprecher 80 

Testing of Enemy Signal Infrared Photocells 

Photocells for Lichtsprechers 250 and 80 

Navy Projects 

Equipment for the Detection of Night Landing Parties 

Thermal Detector with Remote Indicator 

JAPIR Detection Equipment 

Plane-to-Plane Recognition 

Long Wavelength Infrared Range Finder 

Angular Rate Bomb Release System, Employing a Long Wavelength NAN Detecting System 
Iradar 

Infrared Locator for Installation on Submarines for Detection of Ships 
Infrared Receivers and 2 Modulated Infrared Sources 
Development of Infrared Transmitting Filters 

Development of Thermopiles with High Speeds of Response _ . ^ . . 

Development of Infrared Signalling Equipment for Use in Ship-to-Ship Communication 



385 











SERVICE PROJECT NUMBERS ( Continued) 


Service Project Number 

Title 

NS-161 

NS-163 

NS-163 (Revised) 

NS-181 

NS-187 

NS-225 

NS-243 

Development of a High-Speed Thermistor Bolometer 

Measurement of Thermal Changes at Horizon 

Thermal Radiation Ship and Background Survey 

Gyrostabilized Ship Detector 

Infrared Signalling Equipment with Photoelastic Shutter for Ship-to-Ship Communication 
Thallous Sulfide Photoconductive Cells 

Development of a Voice or Morse Modulated Infrared Beam Communication Transmitter 
and Receiver 

NS-371 

Optically Modulated Voice Communication System Over Infrared or Ultraviolet Radiation 
Beams 


386 







INDEX 


The subject indexes of all STR volumes are combined in a master index 
access to the index volume consult the Army or Navy Agency listed on 


Absorption, infrared radiation in at¬ 
mosphere, 282-283 
by organic vapors, 283 
by ozone, 282 
by water vapor, 282-283 
Lambert’s law, 283 

Absorption bands, glasses and plastics, 

165 

Absorptivity, far infrared detectors, 
235-236 

Agency for integration of infrared re¬ 
search, need for, 168, 199 
Aircraft communication systems, 134— 

145 

developmental course, 134-135 
plane-to-ground (P-G), 135-140 
plane-to-plane (P-P), 140-145 
Alkali blacks photoemission, 57 
Allocation agency for infrared fre¬ 
quencies, need for, 168, 199 
Andrew superconducting bolometer, 
271-273 

Angle of view theorem, 60 
Antimony, electrical properties, 239- 

241 

“Apparent holoresponsivity,” 366 
“Apparent infraresponsivity,” 366 
Aquadag, thallous sulfide cell material, 
69 

ASE photophone, British systems, 110 
Atmospheric attenuation of infrared 
radiation 

absorption by organic vapors, 283 
absorption by ozone, 282 
absorption by water vapor, 282-283 
attenuation coefficient, 215-216 
fog and clouds as attenuation factor, 
222-224 

Lambert’s law, 283 
operation range dependent on at¬ 
tenuation, 215-216 
scattering by fogs, 283 
ship-fired smokes, 285-286 
summary, 281-282 
“window” in atmosphere, 282-283 
Atmospheric attenuation of infrared 
radiation, measurement, 283-286 
attenuated response curves com¬ 
pared with envelope curves, 285 
bolometer, 283 

envelope of initial radiation, 284 
gaseous screens, 286 
recording system, 283-284 
solar hologram, 285 


source lamp, 283 
spectrometer, 283 

Atmospheric transmission, intermedi¬ 
ate infrared region 
compared with near and far infra¬ 
red transmission, 165 
haze losses, 165 

Lambert’s law, violation of, 164— 
165 

measurement methods, 164 
transmission through smoke, 165 
water vapor transmission, 164 

Atmospheric transmission, limitation 
of wavelength region, 44 

Atmospheric transmission window, 
226 

Attenuation of infrared radiation by 
atmosphere 

see Atmospheric attenuation of in¬ 
frared radiation 

Background noise and signals 
see Thermal radiations of targets 
and background 

Backscatter in voice-code communica¬ 
tion systems, 104 

Backscatter reduction 

compensating device of retrodirec- 
tive target locator (RTL), 194- 
195 

plane-to-plane recognition (PR) 
system, 191 

Bakelite resin BR-0014, bolometer 
cement, 253 

Ballistic photography lamp 
see Microflash lamps for ballistic 
photography 

BARB (British angular rate bomb- 
sight), 326 

Beacons, incandescent tungsten, 14-18 
code transmission, 16 
coding of emitted radiation, 14-15 
radiation modulation, 16-17 
type D beacon, 14-16 
type D-2 beacon, 16-18 

Beep-canceler of portable ship detec¬ 
tor, 330 

Bell Telephone Laboratories 
PND (portable infrared detector), 
290-294 

SND (scanning infrared detector), 
294-302 

Bell Telephone Laboratories, bolom¬ 
eters 


printed in a separate volume. For 
the reverse of the half-title page. 

characteristics summarized, 264 
compared with other detectors, 277 
physical description, 255 
resistance values, 256 
sensitivity values, 261 
spectroscopic recording use, 360-361 
Birefringence in photoelastic shutter 
transmitter, 153-154 
Bismuth, electrical properties, 239-241 
“Black body” radiation, 225-226 
“Black body” temperature, targets vs. 

background, 287-288 
Blind flying system for gliders 
see GPI (glider position indicator) 
Blinker code devices, German, 108 
Bologram, solar, 285 
Bolometers, 247-275 
Donau Geriit bolometer, 274 
Felix bolometer, 266-269 
Italian bolometer, 274-275 
metal strip (Strong) bolometers, 
247-251 

PND (portable infrared detector) 
bolometer, 291 

Polaroid evaporated bolometer, 269- 
270 

RCA bolometer, 270-271 
responsivity, methods of specifying, 
228 

Strong bolometers, 247-251 
summary, 229 

superconducting bolometer, 271-273 
thermistor bolometers, 251-265 
Bolometers, output prediction, 233- 
234 

bridge factor, 234 
frequency factor, 233 
general response equation, 234 
voltage dependence on circuit, 234 
waveform factor, 234 
Bombsight, British BARB, 326 
Bombsight with angular rate release 
(FIRBARR), 326-328 
Brownian noise in infrared detectors, 
229 

CAM (cloud attenuation meter) 
see Cloud attenuation meter 
(CAM) 

Cameras for thermal receiver, 321 
Carbon dioxide absorption band in 
atmosphere, 282 

Carbonyl process, bolometer construc¬ 
tion, 249 


387 





388 


INDEX 


Carrier-wave communication systems, 

145-161 

polarization systems, 150-161 
Signal Corps optiphone, 145 
Touvet system, 146-150 
types of signals, 145 
V-M Corporation system, 145 
Case 

thalofide cells, 61-62 
World War I voice system, 109 
Cashman 

lead sulfide evaporation, 83 
semiconductors, 63 
Cashman thallous sulfide cells 
see TF (thallous sulfide) cells 
“Caspar” equipment, 290-345 
PND (portable infrared detector), 
292-294 

PSD (portable ship detector), 328- 
332 

SND (scanning infrared detector), 
294-302 

SSD (stabilized ship detector), 332- 
345 

summary, 5-6 

thermal receiver with remote indi¬ 
cator, 313-326 

TMR (thermal map recorder), 302- 
313 

Cellulose nitrate, thermopile film, 241- 
242 

Cesium vapor lamp, 38-43 
design and construction, 38-39 
dynamic impedance, 39 
electrical characteristics, 38-40 
infrared filters, 42-43 
life test data, 41 
modulation ratio, 38 
optical systems, applications, 41- 
42 

per cent current modulation, 41 
starting circuits, 39-40 
total flux, 41 
volt-ampere curve, 39 
Cesium vapor lamp, communications 
use 

compared with concentrated arc 
lamp, 123, 131-132 
for plane-to-ground (P-G) com¬ 
munication system, 137 
modulation, 124 
operation, 123-124 
operational tests, 131-132 
Cesium vapor lamp, radiation charac¬ 
teristics, 40-43 

ehT values of filters, 40, 42-43 
spectral energy distribution, 40-41 
total flux, 40 
visual intensity, 40-41 


Cloud attenuation meter (CAM), 
220-224 

atmospheric attenuation, 215-216 
attenuation caused by fog, 222 
attenuation coefficient measurement, 
sources of error, 223-224 
clouds, effect on attenuation, 223- 
224 

conclusions and recommendations. 
224 

control-meter unit, 222 
field tests, 223 
operational theory, 222-223 
reflector-modulator unit, 222 
source detector unit, 220-222 
threshold contrast, 215-216 
visual range, 215-216 
visual range, correlation with at¬ 
tenuation coefficient, 223-224 
Coaxial shutter, mechanical coding 
use, 180 

Coblentz thermopile, radiation source, 
237-238 

Code and identification system type 
D, 172-189 

90-cycle frequency, 179 
compared with ship-to-ship system, 
178-179, 188-189 
daylight signaling. 187 
design, 173-178 

developmental phases. 172-173 
laboratory type D, 174-176 
laboratory type D-2, 175-176 
operational tests, 187 
optimum code and scanning speeds, 
187 

range, 178 
receiver, 184-186 

recommendations for future devel¬ 
opment, 188-189 
security, 178 

ship-to-plane recognition system, 
173-174 

transmitter, 180-184 
Code and recognition communication 
systems, 170-199 
beacons, 14-18 

code and identification system type 
D, 172-189 

Japanese infrared detection 
(JAPIR) system, 196-198 
military applications, 171-172 
modulation requirements, coding vs. 

voice communication, 170-171 
mutual interference between sys¬ 
tems, 199 

plane-to-plane (P-P) system, 189- 
192 

Polaroid Corporation type P sys¬ 
tem, 172 


recommendations for future devel¬ 
opment, 198-199 

retrodirective target locator (RTL), 
192-195 

Code and voice communication sys¬ 
tems 

see Voice-code communication sys¬ 
tems 

Communication system type L, ISO- 
151 

Communication system type W 
see Telephone, infrared optical 

Communication systems 
see Code and recognition communi¬ 
cation systems; Voice-code com¬ 
munication systems 

Communication systems, vibrating 
mirror, 110-117 

German lichtsprechers, 111-112 
Japanese light-beam telephone, 111 
optical telephone, type W, 112-117 
RCA type R-2 unit, 110-111 

Communication systems with electri¬ 
cal modulation, 117-145 
aircraft systems, 134-135 
plane-to-ground (P-G) system, 134- 
140 

plane-to-plane (P-P) system, 140- 
145 

ship-to-ship system, 118-140 
Western Union aural signal unit, 
117 

Concentrated-arc lamp, 26-38 
applications, 26-27 
cathode spot, 27-29, 31-32 
design and construction, 27-29 
ehT values of filters, 37 
infrared filters, 37 
lamp life, 31-32 
lamp lives when modulated, 36 
operating data, 27 
radiant efficiency, 32 
research recommendations, 38 
spectral distribution, 32-33 
static radiation characteristics, 31- 
33 

summary, 37-38, 43 
wide-beam optical system applica¬ 
tions, 36-37 

Concentrated-arc lamp, communica¬ 
tions use, 122-124 
characteristics, 123 

compared with cesium vapor lamp, 
123, 131-132 
modulation, 124 
night operations tests, 130-131 
operation, 123-124 

Concentrated-arc lamp, electrical char¬ 
acteristics, 29-31, 33-34 
dynamic characteristics, 33-34 





INDEX 


389 


impedance, 33 
modulating circuits, 33-34 
operating circuits, 31 
starting circuits, 30-31 
volt-ampere characteristic, 29-30 
Concentrated-arc lamp, modulated 
radiation, 34-36 
distortion, 36 

modulation characteristics, 34-35 
modulation ratio, 35-36 
per cent modulation, emitted radia¬ 
tion, 36 

radiation components, 34 
spectral distribution, radiation, 35 
Corning glass nacelle filters, 190 

“D” communication system 
see Code and identification system 
type D 

Daylight visible range, communica¬ 
tion systems, 100 
D-c responsivity of thermopiles 
Harris thermopiles, 244 
measurement of thermopile output, 
233 

Schwarz thermopile, 246-247 
Detectors (FIR), 225-278 
absorptivity, 235-236 
bolometers, 247-275 
comparison basis for detectors, 276- 
277 

frequency response, 234-235, 276- 
277 

Golay heat detector, 275-276 
heat radiation in FIR region, 225- 
226 

heating and cooling curves, 234- 
235 

military applications, 225-226 
PND (portable infrared detector), 
292-294 

responsivity, method of specifying, 
228 

spectral response, 235-236 
summary, 278 
terminology, 364-367 
window for radiation transmission, 
235 

zero-frequency responsivity, 277 
Detectors (FIR), noise, 228-232 
amplifying system noise, 230 
Brownian motions of gas particles, 
229 

current noise, 229 
effective noise bandwidth, 229 
ENI (equivalent noise input), 232, 
276 

inherent noise, nature of, 229-230 
Johnson noise, 229-230 


MDS (minimum detectable signal), 
231-232 

mean square noise voltage, 229 
MENI (minimum equivalent noise 
input), 231 

noise output, slow vs. rapid detec¬ 
tion, 230 

“shot effect” of electron fluctua¬ 
tions, 229 

signal noise, 228-229 

Detectors (FIR), test methods, 237- 
239 

amplifiers, 238 

black body as radiation source, 237- 
239 

Coblentz thermopile, flux density 
determinant, 237-238 
frequency response measurements, 
238-239 

heat sources, 237-238 
minimum detectable signal, 239 
Nernst glower as radiation source, 
237 

“parallelogram method” of time con¬ 
stant determination, 238-239 
radiation standard, calibrated lamp, 
237 

spectral response determination, 

239 

Detectors (NIR), 55-92 
military applications, 56 
photoconductive cells, 61-92 
phototubes and photomultipliers, 
56-61 

security and infrared signaling, 56 

Direction and range detecting systems 
see IRRAD 

Directional indication, plane-to-plane 
recognition (PR) system, 190 

Donau Gerat bolometer, 274 

“Drone” (radio-controlled aircraft) 
scanning device 

see Thermal receiver with remote 
indicator 

Dyes for infrared filters, 50-53 
evaporation, 51 

polyvinyl alcohol soluble dyes, 51 
spirit soluble dyes, 53 
stability, vat dyes vs. cellophane, 
50-51 

“E” communication system 

see Ship-to-ship communication 
system 

Effective field of view, HR receiver, 
347-348 

Effective holo-infra response factor, 
366 

Effective hololuminous transmission 
(ehT) 


definition and equation, 45, 368 
filters, infrared, 46 
filters, near infrared, 54 
low evT and high ehT, 47-49 
measurement devices, 46 
Effective infratransmission (eiT), 368 
Effective photo-holo conversion fac¬ 
tor, 364-365 

Effective photo-infra conversion fac¬ 
tor, 365 

Effective visual transmission (evT) 
filters, near infrared, 46-47, 54 
formula, 45 

low evT and high ehT, 47-49 
measurement, 47 
ehT (effective holotransmission) 
definition and equation, 45, 368 
filters, infrared, 46 
filters, near infrared, 54 
low evT and high ehT, 47-49 
measurement devices, 46 
eiT (effective infratransmission). 368 
Elac, German lead sulfide cells, 83- 
84 

ENI (equivalent noise input) 
comparison basis for detectors, 276 
equations, 263-264 
far infrared detectors, 230-232 
Harris thermopiles, 244 
Schwarz thermopiles, 247 
Strong bolometers, 250-251 
Eppley thermocouple, 246, 277 
Equations 

“black body” radiation, 225 
bolometer general response, 234 
effective beam candlepower, 101-102 
evT (effective visual transmission), 

47 

range of voice-code communication 
systems, 99, 105 
Stefan’s law, 237 

TF (thallous sulfide) cell response 
to flux, 79 

thermopiles responsivity, 233 
threshold flux of voice-code com¬ 
munication systems, 103 
voltages for infrared detectors, 229 
Equations, thermistor bolometers, 
262-265 

ENI (equivalent noise input), 263- 

264 

final steady-state sensitivity, 264- 

265 

MENI (minimum equivalent noise 
input), 263-264 

temperature rise above ambient 
temperature, 263 

thermistor resistance relation. 263 
voltage output per watt incident 
radiation, 262-263 




390 


INDEX 


Equivalent hololumens, definition and 
equation, 364-365 
Equivalent infralumens, 365 
Equivalent noise input (ENI) 
comparison basis for detectors, 276 
equations, 263-264 
far infrared detectors, 230-232 
Harris thermopiles, 244 
Schwarz thermopiles, 247 
Strong bolometers, 250-251 
Esterline-Angus recorder, 239 
Evaporation of metals, thermopile 
construction, 239-240 
evT (effective visual transmission) 
filters, near infrared, 46-47, 54 
formula, 45 

low evT and high ehT, 47-49 
measurement, 47 

Fading effect in infrared transmission, 
323-326 

Far infrared atmospheric transmis¬ 
sion, 165 

Far infrared detectors 
see Detectors (FIR) 

Far infrared filters, 54 
Fax recording paper, 336 
Felix bolometer, 265-269 
characteristics summarized, 267 
compared with other detectors, 277 
construction, 265-266 
frequency response, 268 
output voltage, 269 
resistance, 265-266 
spectral response, 266-267 
Filament lamp communication sys¬ 
tems, 110 
Filters 

far infrared communication, 54 
intermediate infrared communica¬ 
tion, 166 
summary, 2-3 

Filters for near infrared transmission 
see Transmission filters (NIR) 

FIR (far infrared) atmospheric trans¬ 
mission, 165 

FIR (far infrared) filters, 54 
FIRBARR bombsight, 326-328 
Flash lamps, 18-26 
early designs, 18 

microflash lamp type, 21-26, 71, 105 
Flash lamps, high-intensity, 19-21 
beam holocandle power, 20-21 
design, 19 

duration of flash, 19-20 

ehT value, 20-21 

lamp life, 20-21 

method of firing, 19 

peak intensity of flash, 19-20 


Fog 

attenuation of infrared radiation, 
222-223, 286 
particle size, 283 

penetration by intermediate infra¬ 
red, 85 

scattering of infrared radiation, 283 
“Footlight effect” of ship exposed to 
water’s radiation, 290 
14PEI phototube, 59 
Fractional transmission of filters, 45 
Frequency allocation for infrared com¬ 
munication devices. 168, 199 
Frequency shift coding, 183 
Fresnel lenses 

for code and identification system 
type D, 180 

for incandescent tungsten beacon, 
15-16 

Galena, rectifying properties, 83 
Gaseous discharge lamps, 18-43 
cesium vapor lamp, 38-43 
concentrated-arc lamps, 26-38 
flash lamps, 18-26 

Gases, attenuation of infrared radia¬ 
tion, 286 

Gelatin filters, dyed, 49 
German lead sulfide cells, 83-84 
German thallous sulfide cells, 62 
Glasses, absorption bands, 165 
Glider position indicator 
see GPI (glider position indicator) 
Golay heat detector 
compared witth other detectors, 277 
construction and operation, 275-276 
responsivity, 228 
summary, 227-228 

GPI (glider position indicator), 215- 
220 

atmospheric attenuation, 215-216 
directional vs. positional systems, 

215 

flight tests, 219 
operation, 216-217 
receiver, 218 

recommendations for future devel¬ 
opment, 220 

sensitivity, field test measurements, 
218-219 
summary, 12 
transmitter, 217 

visual range and attenuation, 215— 

216 

Guided missile, illumination by flash 
lamp, 19 

Harris thermopiles, 239-245 
backing support for metals, 241 
cellulose nitrate films, 241-242 


d-c responsivities, 244 
ENI (equivalent noise input), 244 
evaporated antimony and bismuth, 
electrical properties, 239-241 
evaporation frames, 242 
evaporation method, metals, 239-240 
frequency response, 244-245 
physical dimensions and properties, 
243 

stacking and mounting, 242-243 
Haze 

particles, 283 

penetration by intermediate infra¬ 
red, 85 

solar radiation scattering, 165 
Heat detection 
lead sulfide cells, 88-89 
TF (thallous sulfide) cells, 76 
use of intermediate infrared, 85 
Heat radiation in far infrared region, 
225-226 

Heat-homing targets, airborne survey 
see TMR (thermal map recorder) 
Hololumen, definition and equation, 
364 

Hololumen systems, near infrared 
photometry nomenclature, 9-10 
Holoresponsivity of detector or re¬ 
ceiver, 366 

Hubbert’s notations on sea reflections, 
289 

Identification communication system 
type D 

see Code and identification system 
type D 

IIR (intermediate infrared) 
developments summarized, 7 
haze and fog penetration, 85 
heat detection use, 85 
security greater than near infrared, 
85 

IIR atmospheric transmission 
compared with near infrared trans¬ 
mission, 165 
haze losses, 165 

Lambert’s law, violation, 164-165 
measurement methods, 164 
transmission through smoke, 165 
water vapor transmission, 164 
IIR communication systems, 162-168 
advantages and disadvantages, 162- 
163 

atmospheric transmission in inter¬ 
mediate infrared region, 164-165 
compared with near infrared sys¬ 
tems, 167-168 
filters, 166 

gas discharge source, 166-167 
interference, 167-168 



INDEX 


391 


lead sulfide cells as detectors, 163 
mechanical modulation source, 166- 

167 

optical elements, glass and plastic, 

165 

range, 167-168 
transmission curves, 166 
HR filters, 53, 166 
IIR photosensitive detectors 
see Lead sulfide cells 
IIR receiver, 347-351 
amplifier, 349 

compared with stabilized ship de¬ 
tector (SSD), 350 
detecting element, 348-349 
operation, 347 
optical system, 347-348 
personnel detection tests, 349-350 
recommendations for future devel¬ 
opment, 351 

scanning mechanism, 348 
ship detection tests, 350 
threshold sensitivity, 349 
Incandescent tungsten beacons, 14-18 
automatic continuous coder, 15 
“break-in” by receiving operator, 16 
code speed for type D-2 beacon, 18 
coding of emitted radiation, 14 
construction, 16-18 
plane-to-plane recognition system 
use, 18 

radiation modulation, 16-17 
transmission speed for type D 
beacon, 14 

Incandescent tungsten lamps, 11-18 
communication system with photo¬ 
electric shutters, 11-12 
glider position indicator, 12 
infrared optical telephone, 13-14 
microfilm source for photocell set, 14 
modulation difficulties, 10 
plane-to-plane recognition use, 18 
recognition and code communica¬ 
tion beacons, 14-18 
retrodirective reflector target loca¬ 
tor, 12-13 

Infracandle (ic.), definition, 365 
Infralumen, definition, 364 
Infrared component, measurement, 365 
Infrared frequencies, need for alloca¬ 
tion agency, 168, 199 
Infrared research, need for integrat¬ 
ing agency, 168, 199 
Infrared signaling, 56 
Infraresponsivity of detector or re¬ 
ceiver, 366 

Inrush keyers, incandescent tungsten 
lamps, 182-183 
Intermediate infrared 

see IIR 


IRRAD (infrared range and direction 
detecting), 200-214 

IRRAD for diffusely reflecting targets, 
211-214 

atmospheric backscatter, 213 
evaluation, 214 
field tests, 212-214 
photomultiplier tubes, 214 
range of detection for ships, 213-214 
receiver optical system, 212 
retrodirective reflectors, 214 
scanning unit, 212 
ship reflectance, 213 
transmitter optical system, 211-212 
transmitter visual range, 214 
triple mirror vs. large extended tar¬ 
get, 211 

IRRAD for retrodirective reflector 
targets, 200-211 
a-c power supply, 209 
compared with radar, 200 
d-c power supply, 208-209 
field tests, 209-210 
final models, 210-211 
recommendations for future devel¬ 
opment, 211 

source, tungsten vs. mercury, 210 

IRRAD for retrodirective reflector 
targets, components, 201-208 
brightening-voltage generator, 201 
cathode-ray tube and control cir¬ 
cuits, 206-207 

coincidence range circuit, 207 
range index pulse, 206 
range potentiometer, 208 
receiving optical system, 205-206 
scanning head, 201 
signal amplifier, 206 
start-stop range circuit, 207 
sweep amplifier - inverter circuit, 
207 

target, 205 

time-delay range circuit, 207 
transmitting optical system, 203- 
205 

Italian bolometer, 274-275 

Italian photophones, 110 

Japanese light-beam telephone, 111 

JAPIR (Japanese infrared) detection 
system, 196-198 

amplifier and preamplifier, 197-198 
cell arrangement, 197 
chopper orientation, 197 
control panel, 198 
design, 196-197 
evaluation, 197 
operational tests, 198 
recommendations for future devel¬ 
opment, 198 


sky light and moonlight, effect on 
operation, 197 
summary, 172 
test lamp, 198 
threshold sensitivity, 197 

Johnson noise 

definition and summary, 281 
in far infrared detectors, 229-230, 
276 

thallous sulfide cell, frequency re¬ 
sponse theory, 80-81 

Keyer with saturable core reactor, 
182-183 

Lambert’s law 

infrared radiation absorption in 
atmosphere, 283 

yiolation in intermediate infrared 
region, 164-165 

Lamps 

cesium vapor, 38-43 
concentrated-arc, 26-38 
filament, 110 
flash, 18-26 

incandescent tungsten, 11-18 

Landing craft detection 
portable infrared detector (PND), 
294 

portable ship detector (PSD), 328- 
332 

retrodirective target locator. 171 
stabilized ship detector (SSD), 336- 
337 

Lead sulfide cells, 83-90 
as motor exhaust detectors, 89 
cell types, 85 
construction, 86 
German cells, 83-84 
heat detection, 88-89 
holotransmissions, 166 
intermediate infrared detection, 163, 
167-168 

military value, 85 

recommendations for future devel¬ 
opment, 90 

theoretical studies, need for, 90 
variables in preparation process, 89- 
90 

Lead sulfide cells, characteristics, 86- 
89 

daylight range estimates, 141-143 
frequency response, 87-88 
linearity, 88 
resistance, 86 
sensitivity, 163 
signal-to-noise ratio, 84 
signal-to-noise ratio, compared with 
TF cells, 86-87 
spectral response, 86-87 








392 


INDEX 


suitability for intermediate infrared 
use, 84 

theoretical limitations, 88 
variation of noise with voltage, 87 
Lead sulfide cells, receiver, 345-351 
amplifier, 349 

compared with stabilized ship de¬ 
tector (SSD), 350 
detecting element, 348-349 
operation, 347 
optical system, 347-348 
personnel detection tests, 349-350 
recommendations for future devel¬ 
opment, 351 

scanning mechanism, 348 
ship detection tests, 350 
threshold sensitivity, 349 
Lichtsprecher communication systems 
design ingenuity, 111-112 
lead sulfide cells, 83-84 
narrow-beam factor, 108 
vibrator mirror systems, 111-112 
Life jackets equipped with retrodirec- 
tive target locator, 171 
Life raft search equipment 
see RTL (retrodirective target loca¬ 
tor) 

Map of heat-homing targets 
see TMR (thermal map recorder) 
MDS (minimum detectable signal), 
231-232 

Melmac plastic filter, 52-53 
MENI (minimum equivalent noise 
input) 

definition, 231 
equations, 263-264 
Metal strip bolometers, 247-251 
compared with other detectors, 277 
construction, 248-249 
nickel carbonyl construction process, 
249 

nickel strips, production, 248 
operation, 247-248 
windows, 248-249 

Metal strip bolometers, characteristics, 
250-251 

absorptivity, 250 

ENI (equivalent noise input), 250- 

251 

frequency response curve, 250 
output voltages, 251 
ratio of bolometer response to 
Coblentz response, 250 
resistance, 249-250 
spectral response, 250 
Meteorological conditions, effect on 
target infrared radiation emis¬ 
sion, 289-290 


Microbeacons, test devices for voice- 
code communication systems, 

106-107 

Microflash lamps, 21-26 
IRRAD components, 203-206 
type 200; 21-23 
type 300 ; 23-26 

Microflash lamps for ballistic photog¬ 
raphy, 21-23 
design, 21-22 
duration of flash, 22 
firing method, 22 
lamp life, 22 
microflash unit, 22 
peak intensity of flash, 22 
performance, 23 

Microflash lamps for target detection, 
23-26 

design, 23-24 

duration of flash, 25-26 

ehT values of filters, 25-26 

firing method, 24-25 

firing rate, 26 

flashing rate, 25 

lamp life, 26 

peak intensity of flash, 25 
Microflux source, photocell test set, 

14 

Minimum detectable signal (MDS), 
231-232 

Minimum equivalent noise input 
(MENI) 
definition, 231 

equations for computation, 263-264 
Mirror communication systems, 110— 

117, 200-214 

German Lichtsprechers, 111-112 
IRRAD for diffusely reflecting tar¬ 
gets, 211-214 

IRRAD for retrodirective reflector 
targets, 200-211 

Japanese light-beam telephone, 111 
optical telephone, 112-117 
RCA type R-2 unit, 110-111 
Mirrors 

retrodirective triple, 192 
vibrating mirror for portable infra¬ 
red detectors (PND), 291-292 
MIT Heat Research Laboratory, bo¬ 
lometers 

see Felix bolometer 
MK23 bombsight, 326-327 
Modulated filament lamp communica¬ 
tion systems, 110 
British systems, 110 
Italian photophones, 110 
voice modulation, disadvantage, 110 
Modulated tungsten lamp, 166-167 
Modulation methods 


coding vs. voice requirements, 170- 
171 

voice-code communication systems, 
96-97 

Modulation ratios, near infrared 
sources, 10-11 

Moonlight, effect on operation of 
JAPIR detection system, 197 
Motion picture 

optical and sound communication 
medium, 109 » 

vibrating mirror for sound-recording 
system, 115 

Motor exhaust detection by lead sul¬ 
fide cells, 89 

Mt. Wilson retrodirective triple 
mirrors, 192 

‘‘Nancy” military equipments 
GPI, 215-220 
IRRAD, 200-214 
JAPIR, 196-198 

life raft search equipment, 192-195 
recognition and code communication 
system, type D, 172-189 
summary, 3-5 

voice and code communication sys¬ 
tems, 96-169 

Navigation markers, possible use of 
retrodirective target locator, 171 
Near infrared 
see NIR 
Nernst glower 
absorptivity measures, 236 
radiation source, 44, 237 
Nickel carbonyl process, bolometer 
construction, 249 
Nickel strips for bolometers, 248 
Night bombing planes, detection by re¬ 
flected infrared radiation, 62-63 
Night surveying by infrared 

see IRRAD (infrared range and 
direction detecting) 

NIR (near infrared), compared with 
intermediate infrared, 84-85 
NIR atmospheric transmission, 165 
NIR detecting devices, 55-92 
military applications, 56 
photoconductive cells, 61-82 
phototubes and photomultipliers, 
56-61 

security of infrared signaling, 56 
NIR sources, 9-44 
applications, 9 
cesium vapor lamp, 38-43 
comparison, 43 
concentrated-arc lamp, 26-38 
incandescent tungsten lamps, 11-18 
microflash lamps, 21-26 
modulation ratio, 10-11 





INDEX 


393 


nomenclature, 9-10 
recommendations for future devel¬ 
opment, 43-44 
summary, 1-2 

Noise (holo or infra) threshold of de¬ 
tector or receiver, 367 
Nonex, thallous sulfide cell material, 
69 

Normal visual range (NVR), 47 
Nylon infrared filters, 51 

Operational (holo or infra) threshold 
of receiver, 367 

Optical communication transmission 
vs. voice transmission, 93 
Optiphone, 145, 153 
Ozone absorption band in atmosphere, 
282 

Per cent current modulation, 11 
Personnel detection 
lead sulfide cells, 349-350 
TMR (thermal map recorder), 309 
Pfund’s experiments on transmission 
in infrared, 261 
P-G communication system 
see Plane-to-ground (P-G) com¬ 
munication system 
Photocell test set 
microflux source, 14 
TF (thallous sulfide) testing, 72-74 
Photoconductive cells, 61-82 
lead sulfide cells, 83-90 
photoconductive process, 78, 82-83 
properties summarized, 66 
recommendations for future devel¬ 
opment, 92 

selenium electrolytic cells, 92 
silicon cells, 90-92 
TF (thallous sulfide) cells, 61-83 
Photoelastic shutter communication 
system, 151-160 
design, 151-153 
developmental course, 151 
evaluation, 153 
monitor, 159 

operational tests, 159-161 
range, 153 
receiver, 159 

recommendations for future devel¬ 
opment, 160 
security, 153 
summary, 11-12 

variable quarter-wave plate, 159 
Photoelastic shutter communication 
system, transmitter, 153-159 
amplitude optimum, 154 
birefringence, optimum conditions 
for, 153-154 

comparison with optiphone, 153 


driving crystals, 154, 158 
glass blocks, 154, 158 
oscillator, 159 

soldering-quartz and glass, 158-159 
stripped shutter operation, 157-158 
uniform shutter operation, 154-157 
Photoemission from alkali blacks, 57 
Photographic recordings with remote 
control thermal receiver, 321— 
323 

Photographs of target areas from 
planes, 308 

Photometry, near infrared, 9-10 
Photomultipliers 

see Phototubes and photomultipliers 
Photophones, British, 110 
Photophones, Italian, 110 
Phototubes and photomultipliers, 56- 

61 

angle-of-view study, 60 
course of development, 57 
IRRAD use, 205-206 
noise characteristics, 57-58 
optimum voltage per stage, 61 
photoemission from alkali blacks, 
57 

recommendations for future devel¬ 
opment, 61 

ship-to-ship communication system 
use, 126 

theoretical studies, 60-61 
Phototubes and photomultipliers, con¬ 
struction, 57-60 
cesium cathode surfaces, 57-58 
fluorescence reduction, 59 
fourteen stage tubes, 58-59 
quadrant multiplier, 60 
side view tubes, 59 
signal-to-noise ratio, 58 
six-stage end-view multipliers, 58 
ten stage tubes, 59 
vacuum phototubes, 60 
Photovoltaic effects of TF (thallous 
sulfide) cells, 76-77 
Planck’s equation, black body emis¬ 
sion, 225 

Plane-to-ground (P-G) communica¬ 
tion system, 135-140 
cesium vapor lamp, 137 
design and construction, 135-136 
directivity patterns of the optical 
system, 138 
evaluation, 136-137 
filter, 138 

ground unit; see Telephone, infrared 
optical 

operational tests, 138-140 
range, 136 
receiver, 137-138 


requirements for plane and ground 
units, 134 

ripple noise elimination, 138 
security, 136 

Plane-to-plane (P-P) communication 
system, 140-145 
control panel, 143 
design, 140-141 
evaluation, 142-143 
operational tests, 144 
optical systems, 144 
range, estimated, 141-143 
receiver, 143-144 

recommendations for future devel¬ 
opment, 144-145 
security, 141-142 
transmitter, 143 

Plane-to-plane recognition (PR) sys¬ 
tem, 189-192 
90-cycle modulation, 191 
backscatter reduction, 191 
design, 189-190 
development, 189-190 
directional indication, 190 
incandescent tungsten lamp, 18 
positional indication, 191 
range, 190 
receiver, 191-192 

recommendations for future devel¬ 
opment, 192 

resistance balance maintenance, 191 
security, 190 
transmitter, 191 

Plane-to-ship identification, 189-190 
Plastic filters, dyed, 50-52 
evaporated filters, 51 
fabrication from dyed cellophane 
sheet, 50 

heat resistance, 50-51 
nylon filters, 51 
polyvinyl alcohol filters, 51 
Plastics, absorption bands, 165 
PND (portable infrared detector), 
292-294 

ambient temperature, operational 
limitation, 293 

background compensation, 294 
bolometer, 291 
components, 291-292 
development, 290 

effect of weather on performance, 
293 

indicator units, 292 
minimum signal, 294 
operation, 292-293 
optical system, 291 
orientation, horizontal vs. vertical, 
293 

radiation accepted by apparatus, 
292-293 



394 


INDEX 


range, 294 

summary, 6, 279-280 
viewing through windows, 293 
Polarization communication systems, 
150-161 

photoelastic shutter system, 151-160 
type L system, 150-151 
types of systems, 150 
Polarization of light, photoelastic 
shutter communication system, 
154-158 

birefringence variations, 153-154 
quarter-wave plate replaced by 
phase difference plates, 159 
stripped shutter case, 157-158 
uniform polarizer case, 154-157 
Polaroid Corporation 
code and recognition system, type P, 
172 

evaporated bolometer, 269-270 
Polaroid Corporation, infrared filters, 
50-52 

evaporated filters, 51 
fabrication from dyed cellophane 
sheet, 50 

heat resistance, 50-51 
nylon filters, 51 
polyvinyl alcohol filters, 51 
ship-to-ship communication system 
use, 129 

Polyvinyl alcohol (PVA) base filters, 

51 

Polyvinyl butyral, bolometer binder, 
252 

Position indicator for gliders 
see GPI (glider position indicator) 
P-P communication system 
see Plane-to-plane (P-P) communi¬ 
cation system 
PR recognition system 
see Plane-to-plane recognition (PR) 
system 

PSD (portable ship detector), 328- 
332 

amplifier, 330 
beep-canceler, 330 
bolometer bridge, 330 
compared with stabilized ship de¬ 
tector (SSD), 332 
detecting element, 329-330 
detection ranges, 331 
minimum detectable signal, 330-331 
operation, 328 
optical system, 328 
power supply, 330 
presentation unit, 330 
scanning system, 328-329 
summary, 5 

PYA (polyvinyl alcohol) filters, 51 


Quadrant photomultiplier, tracking 
equipment, 60 

R-2 vibrating mirror communication 
system, 110-111 

Radar principles, compared with 
IRRAD, 200 

Radiation, far infrared heat, 225-226 

Radiations, targets vs. background 
see Thermal radiations of targets 
and background 

Radio-controlled aircraft, scanning 
device 

see Thermal receiver with remote 
indicator 

Range and direction detecting systems 
see IRRAD 

Range equation, voice-code communi¬ 
cation, 99, 105 

Range finder (FIR), 351-360 
automatic following, 352, 359-360 
field tests, 357 
operation, 351-353 
operational tests, 359 
tests on controlled target, 356-360 

Range finder (FIR), components, 353- 
356 

amplidyne control circuit, 356 
amplifier circuit, 355 
amplifier system, 355-356 
bolometer bridge, 355 
bolometers, 353-355 
optical system, 353 
phasemeter circuits, 355-356 
presentation unit, 356 

RCA bolometer, 270-271 

RCA type R-2 vibrating mirror com¬ 
munication system, 110-111 

Receiving systems (FIR), 279-361 
bombsight with angular rate release 
(FIRBARR), 326-327 
lead sulfide cell, 345-351 
PND (portable infrared detector), 
290-294 

PSD (portable ship detector), 328- 
332 

range finder, 351-360 
SETT (spectrophotometric ele¬ 
ment), 360-361 

SND (scanning infrared detector), 
294-302 

SSD (stabilized ship detector), 332- 
345 

summary, 279-280 

thermal receiver with remote indi¬ 
cator, 313-326 

TMR (thermal map recorder), 302- 
313 

Receiving systems (FIR), common as¬ 
pects, 280-290 


amplifiers, 281 

atmospheric attenuation of infrared 
radiation, 282-286 
detectors, 281 
military applications, 279 
optical systems, 281 
terminology, 364-367 
thermal radiations from targets and 
backgrounds, 286-290 
Recognition communication systems 
see Code and recognition communi¬ 
cation systems 

Recommendations for future develop¬ 
ment 

code and recognition communica¬ 
tion systems, 198-199 
GPI (glider position indicator), 220 
IRRAD for retrodirective reflector 
targets, 210-211 

lead sulfide cell (HR) receiver, 351 
lead sulfide cells, 90 
near infrared (NIR) sources, 43-44 
optical telephone, infrared, 117 
photoconductive cells, 92 
photoelastic shutter communication 
system, 160 

plane-to-plane (P-P) communica¬ 
tion system, 144-145 
plane-to-plane recognition (PR) 
system, 192 

ship-to-ship communication system, 
133-134 

TMR (thermal map recorder), 310- 
313 

Touvet communication system, 150 
voice-code communication systems, 
168 

Relative spectral radiant flux of filters, 
45 

Relative special responsivity, 45 
Retrodirective reflectors, possible 
IRRAD targets, 214 
RMU (reflector-modulator unit) of 
cloud attenuation meter, 222 
Rock salt window, bolometers, 250 
RTL (retrodirective target locator), 
192-195 

applications, 171-172 
backscatter compensating device, 
194-195 

daytime identification, 195 
design, 192-194 
development, 192 
double attenuation feature, 194 
evaluation, 194 
operational tests, 195 
range, 194 
security, 193-194 

snow and rain effect on operation, 
195 




INDEX 


395 


summary, 12-13 
telescopic RTL, 194 
triple-mirror reflectors, 192-193 

Schwarz thermopile, 246-247 
compared with other detectors, 277 
construction, 246-247 
d-c responsivity, 246-247 
equivalent noise input (ENI), 247 
frequency response, 247 
Scofoni communication system, 145, 
153 

SDO (source-detector unit) of cloud 
attenuation meter, 220-222 
Sea and sky temperatures, 289 
Sea vapors, absorption of infrared ra¬ 
diation, 283 

Selenium electrolytic cells, 92 
SETT, spectrophotometric element, 
360-361 
Ship detection 

IRRAD (infrared range and direc¬ 
tion detecting), 200-214 
PSD (portable ship detector), 328- 
332 

SND (scanning infrared detector), 
294-302 

Ship-to-plane recognition system 
type D, 173-174 

Ship-to-ship communication system, 
118-140 

amalgamation with identification 
system type D, 179, 188-189 
compared with code and identifica¬ 
tion system type D, 178 
developmental history, 118 
evaluation, 122 

interference with identification sys¬ 
tem type D, 179, 188-189 
interrelations of components, 129- 
130 

laboratory model, 118-120 
production models, 121-122 
range, 122 
receiver, 126-127 

reception of code from Touvet sys¬ 
tem, 149 

recommendations for future devel¬ 
opment, 133-134 
security, 122, 134 

Ship-to-ship communication system, 
optics, 127-129 
filters, 129 

phototube receiver, 128-129 
point source transmitter, 127 
TF cell receiver, 129 
vapor lamp transmitter, 127-128 
Ship-to-ship communication system, 
tests, 130-133 

cesium vapor lamp system, 131-132 


code transmission, 132 
concentrated arc system, 130-131 
gunfire, 133 

movement of ships, 132 
searchlights, 133 

Ship-to-ship communication system, 
transmitter, 122-126 
amplification, 124-125 
cesium vapor lamp, 123 
concentrated-arc source, 122-123 
lamp modulation, 124 
lamp operation, 123-124 
lamp starting, 124 
transmitter circuits, 125-126 
“Shot effect/’ electron emission, 229 
Shutter communication system, photo¬ 
elastic 

see Photoelastic shutter communica¬ 
tion system. 

Signal Corps optiphone, 145, 153 
Silicon cells, 90-92 
construction, 91 
developmental history, 90 
need for further study, 92 
properties, 91-92 

resistance variation with tempera¬ 
ture, 91 

signal and noise responses, 91 
spectral response, 91 
Silver chloride, bolometer window ma¬ 
terial, 248-249 

Sky light, effect on operation of 
JAPIR detection system, 197 
Sky temperature, 289 
Smoke 

attenuation of infrared radiation, 
measurements, 285-286 
effect on intermediate infrared trans¬ 
mission, 165 

SND (scanning infrared detector), 
294-302 

airborne tests, 299-302 
development, 294-295 
operation, 295 

ship detection, field tests, 299 
summary, 6, 279-280 
tank detection, field tests, 297-299 
SND (scanning infrared detector), 
components, 296-297 
amplifiers, 297 
detecting element, 296-297 
indicating unit, 297 
optical system, 296 
pass band in amplifier, 299 
scanning mechanism, 296 
Solar radiation, transmission through 
atmospheric water vapor, 164— 
165 

Spectral modulation communication 
system, 161-162 




Spectral response 
far infrared detectors, 235-236 
lead sulfide cells, 86-87 
TF (thallous sulfide) cells, 66, 75 
Spectral transmission, filters, 368 
Spectrometer for recording infrared 
spectra, 360-361 

Spectrophotometric element (SETT), 
360-361 

SSD (stabilized ship detector), 332- 
345 

compared with lead sulfide cell re¬ 
ceiver, 350 

compared with portable ship de¬ 
tector (PSD), 332 
“definite signal range,” 339 
military requirements, 332 
operation, 332, 342 
Receiver responsivity increase, 339 
summary, 5-6 
tests, 337-345 

360° unattended search, 339-342 
SSD (stabilized ship detector), com¬ 
ponents, 333-336 
detecting element, 334-336 
fields of view, 333 
final amplifier, 336 
operational tests, LCI as targets, 
336-337 

optical system, 333 
power supply unit, 335-336 
preamplifier, 335 
representation unit, 336 
scanning mechanism, 333-334 
Stefan’s law, flux densities vs. tem¬ 
perature differences, 237 
Strong bolometer, 247-251 
absorptivity, 250 

compared with other detectors, 277 
construction, 248-249 
ENI (equivalent noise input), 250- 
251 

frequency response curve, 250 
nickel carbonyl construction process, 
249 

nickel strips, production, 248 
operation, 247-248 
output voltages, 251 
ratio of bolometer response to Cob- 
lentz response, 250 
resistance, 249-250 
spectral response, 250 
windows, 248-249 

Submarine equipment for ship detec¬ 
tion. 

see PSD (portable ship detector) 
Sun’s surface brightness, 225 
Superconducting bolometer, 271-273 
Survivors at sea, detection by retro- 
directive target locator, 171 



396 


INDEX 


Tank detection 

scanning infrared detectors (SND), 
297-299 

thermal map recorder (TMR), 310 
Target detection with microflash 
lamps, 23-26 
design, 23-24 
duration of flash, 25-26 
ehT values of filters, 25-26 
firing method, 24-25 
firing rate, 26 
flashing rate, 25 
lamp life, 26 

peak intensity of flash, 25 
Target locator, retrodirective 
see RTL (retrodirective target lo¬ 
cator) 

Targets as thermal radiators 
see Thermal radiations of targets 
and backgrounds 

Telephone, infrared optical, 112-117 
as ground unit for plane-to-ground 
(P-G) communication system, 
135-136, 138-140 
design, 112-114 
evaluation, 114 
operational tests, 116-117 
overmodulation effect on vibrating 
mirror, 115 
range, 114 
receiver, 114-116 

recommendations for future devel¬ 
opment, 117 
security, 114 
specifications, 112 
summary, 13-14 
transmitter, 112-113, 115 
vibrating mirror, 115 
Telephone, Japanese light-beam, 111 
Teletype writer-code communication 
system, 172 

TF (thallous sulfide) cells, 61-83 
code and identification system type 
D, 184 

compared with photoemissive detec¬ 
tors, 68-69 

daylight range estimates, 141-143 
intermediate infrared detection, 166- 
167 

plane-to-ground (P-G) communi¬ 
cation system use, 135-136 
ship-to-ship communication system 
use, 126-127, 129 

TF (thallous sulfide) cells, develop¬ 
ment, 61-65 
British cells, 62 
Case cells, 61-62 
Cashman’s work, 63 
cell property studies, 63 
German cells, 62 


ideal cell characteristics, 63 
instability of early cells, 61-62 
measures of sensitivity, 61-62 
military problems leading to TF cell 
research, 62-63 
production history, 64-65 
TF (thallous sulfide) cells, manufac¬ 
ture, 69-74 

Aquadag component, 69 
cell bodies ,construction, 65-66, 69 
commercial production, 71-72 
contaminating vapors, 71 
effect of impurities, 70-71 
Nonex construction, 69 
oxidation effects, 70-71 
photocell test set, 72-74 
testing procedures, 72-74 
thallous sulfide preparation, 70-71 
variables in manufacturing process, 
70 

TF (thallous sulfide) cells, properties, 
74-77 

see also TF (thallous sulfide) cells, 
theory 

heat detection, 76 
linearity, 75-76 
photovoltaic effects, 76-77 
resistance, 66-68, 74-75 
secondary emission, 76-77 
signal and noise, 67, 75-76 
signal output, 67-68 
spectral response, 66, 75 
summary, 66 

threshold sensitivity, 65, 76 
TF (thallous sulfide) cells, theory, 77- 
83 

crystal structure, 81 
excitation noise, 80 
interrelations amongst properties, 
77-78 

noise-frequency relationship, 78 
phase lag, 81 

photoconductive process, 78, 82-83 
resistance, variations with tempera¬ 
ture, 78-79 

response to single pulse, 79-80 
response to steady flux, 79 
signal and noise, frequency response, 
79-81 

thermoelectric effect, 81 
Thallous sulfide cells 
see TF (thallous sulfide) cells 
Thermal radiations of targets and 
background, 286-290 
emission of targets, 287-288 
“footlight effect,” 290 
measurements of target tempera¬ 
ture, 287 

meteorological conditions, effect on 
emission, 289-290 


origins of background noise and sig¬ 
nal, 288 

sea and sky background, 289 
summary, 282 

summer vs. winter signals, 289-290 
variation vs. fluctuation, 288 
Thermal receiver with remote indi¬ 
cator, 313-326 

airborne installation, 319-321 
fading effect in infrared transmis¬ 
sion, 323-326 
night test flights, 323 
operation, 314-315 
sensitivity, 319 
summary, 280, 313-314 
synchronous photography, targets 
and CRO indications, 321-323 
target approaches, photographic rec¬ 
ords, 321-323 

Thermal receiver with remote indi¬ 
cator, components, 315-319 
amplifiers, 317-318 
bolometer changes, 318-319 
cameras, 321 
carrier frequencies, 318 
modulators for signal channels, 318 
optical system, 315 
preamplifier, 317 
presentation unit, 318 
reflector changes, 319 
scanning mechanism, 315-316 
tilt mechanism, 316 
Thermistor bolometers, 251-265 
absorptivity, 261-262 
Bell Telephone Laboratory bolom¬ 
eters, 255 

ENI (equivalent noise input), 259- 
260 

final steady-state sensitivity, 264-265 
frequency response curves, 256-259 
operation, 251-252 
resistance, 255-256 
sensitivity, 259-261 
spectral response, 261-262 
voltage output, 259-261 
Western Electric amplifier, 254-255 
Thermistor bolometers, construction, 
252—254 

backing process, 253 
firing operation, 252 
flakes, 252-253 

housing base and mount, 253-254 
resistance material, 252 
Thermistor bolometers, equations, 
262-265 

ENI (equivalent noise input), 263- 
264 

MENI (minimum equivalent noise 
input), 263-264 






INDEX 


397 


temperature rise above ambient 
temperature, 263 

thermistor resistance relation, 263 
voltage output per watt incident 
radiation, 262-263 
Thermocouples, 245-246 
Eppley thermocouple, 246 
Weyrich vacuum thermocouple, 245- 
246 

Thermopiles, 239-247 
Harris evaporated thermopile, 239- 

245 

output predicted from d-c respon- 
sivity, 233 

responsivity, methods of specifying, 
228 

Schwarz thermopile, 246-247 
summary, 227 

Weyrich vacuum thermocouple, 245- 

246 

Thirring communication system, 108 
Thyratron-controlled inrush keyer, 
182-183 

TMR (thermal map recorder), 302- 
313 

airborne tank detection tests, 310 
day vs. night operation, 308 
flight tests, 308-309 
general description, 303-304 
high vs. low altitude performance, 
310-313 

military applications, 302-303 
personnel detection tests, 309-310 
photographs of target area, 308 
recommendations for future devel¬ 
opment, 310-313 

scanning speed reduction, 311-313 
summary, 6, 280 

TMR (thermal map recorder), com¬ 
ponents, 304-308 
amplifier, 306 

detecting element, 305-306 
indicating unit, 307-308 
mirror, 304 

scanning and tilt mechanism, 304- 
305 

Touvet communication system, 146— 
150 

design, 146 
evaluation, 147 
French prototype, 146 
photodetector cells, 148 
range, 147 

receiver electronic equipment, 148 
recommendations for future devel¬ 
opment, 150 
security, 147 

ship-to-ship system reception from 
Touvet source, 149 
tests, 149 


transmitter, 147-148 

Transmission filters (NIR), 45-54 
ehT determination, 46 
evT determination, 46-47 
filters for radiation beyond 1.4 me, 
52 

future development, 53-54 
high-density filter requirements, 47- 
49 

low-density filter requirements, 49 
military requirements, 47-49 
Ohio State Research Foundation 
filters, 52-53 
prewar status, 49 
short wave length filters, 52-53 
summary, 2-3, 54 
terminology, 45-47 

Transmission filters (NIR) of Pola¬ 
roid Corporation, 50-52 
evaporated filters, 51 
fabrication from dyed cellophane 
sheet, 50 

heat resistance, 50-51 
nylon filters, 51 

polyvinyl alcohol base filters, 51 

Transmission “window” in atmos¬ 
phere, 226 

Triple-mirror reflectors, detection of 
see IRRAD for retrodirective reflec¬ 
tor targets 

Tungsten lamps, incandescent 
see Incandescent tungsten lamps 

Vacuum phototubes, 60 

Vacuum thermocouple, Weyrich, 245- 
246 

Vapor absorption of infrared radi¬ 
ation, 283 

V-M Corporation communication sys¬ 
tem, 145 

Voice-code communication, 93-169 
aircraft use, 94 

compared with optical communica¬ 
tion systems, 93 
ground use, 94-96 
identification and recognition, 96 
near infrared region, advantages, 93- 
94 

operational principles, 98-99 
range equation, 99, 105 
security, 98 
ship use, 94 

Voice-code communication systems, 
96-169 

aircraft systems, 134-145 
carrier-wave systems, 145-150 
electrically modulated arc systems, 
117-145 

intermediate infrared systems, 162- 
168 


polarization systems, 150-161 
recommendations for future devel¬ 
opment, 168 

spectral modulation system, 161- 
162 

types of systems, 96-98 
vibrating mirror systems, 110-117 
Voice-code communication systems, 
auxiliary equipment, 106-107 
image tubes, 106 

microbeacons, test devices, 106-107 
stabilized platforms, 105-106 
Voice-code communication systems, 
pre NDRC, 107-110 
ASE photophone, British wide-angle 
system, 110 

blinker systems, World War I, 108- 

109 

Case code system, 108-109 
German narrow-beam systems, 108 
Italian photophones, 110 
Lichtsprecher systems, 108 
modulated filament lamp systems, 

110 

sound motion pictures, 109 
Thirring system, 108 
Voice-code communication systems, 
reception, 103-105 
backscatter, 104 
bandwidth, 103 
cell area, 103-104 
daylight operation, 104-105 
effective threshold flux, 103 
laboratory range measurements, 104 
solid angle of view, 103-104 
Voice-code communication systems, 
transmission, 99-103 
atmospheric attenuation, 99-100 
beam solid angle, 102 
filter choice, 102-103 
high efficiency factors, 101-102 
laboratory measurements, 102 
modulation efficiencies, 100-101 
narrow-beam systems, 101-102 
pass band, 101 

rule for choosing source, given prob¬ 
lem, 102 

“W” communication system 
see Telephone, infrared optical 
Water vapor absorption band in at¬ 
mosphere, 282-283 

Water vapor transmission in inter¬ 
mediate infrared region, 164-165 
Lambert’s law violation, 164-165 
scattering on haze, 165 
solar radiation transmission, meas¬ 
urement methods, 164 
Wavelength allocation for infrared 
communication devices, 168 



398 


INDEX 


Western Electric bolometer amplifier, 
254-255 

Western Union aural signal unit, 117 
Weyrich vacuum thermocouple, 245- 
246, 277 


'“Window” in atmosphere for infrared 
transmission, 226 

X-cut driving crystals for photoelastic 
shutter communication system, 
158 


Xenon discharge tube, near infrared 
source, 43 

Xenon lamp, Touvet communication 
system source, 147 
XRX polaroid filter, 50 


















































































